diff --git a/packages/core/src/__tests__/db-migrate.test.ts b/packages/core/src/__tests__/db-migrate.test.ts
index d4fec7f76a..9c1eb7dd74 100644
--- a/packages/core/src/__tests__/db-migrate.test.ts
+++ b/packages/core/src/__tests__/db-migrate.test.ts
@@ -715,7 +715,7 @@ describe("schema migration", () => {
 
     const row = db.prepare("SELECT deletedAt FROM tasks WHERE id = 'FN-legacy'").get() as { deletedAt: string | null };
     expect(row.deletedAt).toBeNull();
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -748,7 +748,7 @@ describe("schema migration", () => {
       { id: "WS-001", mode: "prompt", gateMode: "advisory" },
       { id: "WS-002", mode: "script", gateMode: "advisory" },
     ]);
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -798,7 +798,7 @@ describe("schema migration", () => {
       reviewerContextRetryCount: 0,
       reviewerFallbackRetryCount: 0,
     });
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -827,7 +827,7 @@ describe("schema migration", () => {
 
     const columns = db.prepare("PRAGMA table_info(milestones)").all() as Array<{ name: string }>;
     expect(columns.map((column) => column.name)).toContain("acceptanceCriteria");
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -868,7 +868,7 @@ describe("schema migration", () => {
     const missionColumns = db.prepare("PRAGMA table_info(missions)").all() as Array<{ name: string }>;
     expect(missionColumns.map((column) => column.name)).toContain("autoMerge");
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
     db.close();
   });
 
@@ -902,7 +902,7 @@ describe("schema migration", () => {
       { id: "WS-002", mode: "script", enabled: 1, gateMode: "advisory" },
       { id: "WS-003", mode: "prompt", enabled: 0, gateMode: "advisory" },
     ]);
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -939,7 +939,7 @@ describe("schema migration", () => {
 
     const indexes = db.prepare("PRAGMA index_list(mission_goals)").all() as Array<{ name: string }>;
     expect(indexes.some((index) => index.name === "idxMissionGoalsGoalId")).toBe(true);
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
diff --git a/packages/core/src/__tests__/db.test.ts b/packages/core/src/__tests__/db.test.ts
index 4ad168b50b..344d4c1940 100644
--- a/packages/core/src/__tests__/db.test.ts
+++ b/packages/core/src/__tests__/db.test.ts
@@ -334,7 +334,7 @@ describe("Database", () => {
     });
 
     it("seeds schema version", () => {
-      expect(db.getSchemaVersion()).toBe(105);
+      expect(db.getSchemaVersion()).toBe(106);
     });
 
     it("includes tokenUsageCacheWriteTokens on freshly initialized tasks table", () => {
@@ -393,7 +393,7 @@ describe("Database", () => {
 
     it("is idempotent - calling init() twice does not fail", () => {
       expect(() => db.init()).not.toThrow();
-      expect(db.getSchemaVersion()).toBe(105);
+      expect(db.getSchemaVersion()).toBe(106);
     });
     it("does not overwrite existing config on re-init", () => {
       // Update the config
@@ -1463,7 +1463,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 29 (includes v1→v2 through v26→v29)
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -1488,11 +1488,11 @@ describe("schema migrations", () => {
     const db = new Database(fusionDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     // Re-init should not fail
     db.init();
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     db.close();
   });
@@ -1527,7 +1527,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
     expect(cols.map((col) => col.name)).toContain("priority");
@@ -1568,7 +1568,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
     const colNames = cols.map((col) => col.name);
@@ -1640,7 +1640,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
     const colNames = cols.map((col) => col.name);
@@ -1880,7 +1880,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const cols = db.prepare("PRAGMA table_info(chat_messages)").all() as Array<{ name: string }>;
     expect(cols.map((col) => col.name)).toContain("attachments");
@@ -1954,7 +1954,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const tables = db.prepare("SELECT name FROM sqlite_master WHERE type='table' AND name = 'agentRatings'").all() as Array<{ name: string }>;
     expect(tables).toEqual([{ name: "agentRatings" }]);
@@ -1978,7 +1978,7 @@ describe("schema migrations", () => {
 
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const tables = db.prepare("SELECT name FROM sqlite_master WHERE type='table' AND name = 'mission_events'").all() as Array<{ name: string }>;
     expect(tables).toEqual([{ name: "mission_events" }]);
@@ -2082,7 +2082,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 29
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -2301,7 +2301,7 @@ describe("schema migrations", () => {
 
     localDb.init();
 
-    expect(localDb.getSchemaVersion()).toBe(105);
+    expect(localDb.getSchemaVersion()).toBe(106);
     const columns = localDb.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
     expect(columns.map((column) => column.name)).toContain("tokenUsageCacheWriteTokens");
 
@@ -2612,7 +2612,7 @@ describe("createDatabase factory", () => {
     const db = createDatabase(fusionDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
     expect(db.getLastModified()).toBeGreaterThan(0);
 
     db.close();
@@ -2766,7 +2766,7 @@ describe("migration v77 task token budget columns", () => {
 
       migrated = new Database(fusion);
       migrated.init();
-      expect(migrated.getSchemaVersion()).toBe(105);
+      expect(migrated.getSchemaVersion()).toBe(106);
       const rows = migrated.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
       const names = new Set(rows.map((row) => row.name));
       expect(names.has("tokenBudgetSoftAlertedAt")).toBe(true);
@@ -2812,7 +2812,7 @@ describe("migration v67 drops orphan project auth tables", () => {
 
       migrated = new Database(fusion);
       migrated.init();
-      expect(migrated.getSchemaVersion()).toBe(105);
+      expect(migrated.getSchemaVersion()).toBe(106);
       const tables = migrated
         .prepare("SELECT name FROM sqlite_master WHERE type='table' AND name LIKE 'project_auth_%'")
         .all() as Array<{ name: string }>;
@@ -2839,7 +2839,7 @@ describe("migration v67 drops orphan project auth tables", () => {
 
     try {
       fresh.init();
-      expect(fresh.getSchemaVersion()).toBe(105);
+      expect(fresh.getSchemaVersion()).toBe(106);
       const tables = fresh
         .prepare("SELECT name FROM sqlite_master WHERE type='table' AND name LIKE 'project_auth_%'")
         .all() as Array<{ name: string }>;
diff --git a/packages/core/src/__tests__/goals-schema.test.ts b/packages/core/src/__tests__/goals-schema.test.ts
index ad15bac419..ff887ede36 100644
--- a/packages/core/src/__tests__/goals-schema.test.ts
+++ b/packages/core/src/__tests__/goals-schema.test.ts
@@ -91,6 +91,6 @@ describe("goals schema", () => {
   });
 
   it("reports schema version 101", () => {
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
   });
 });
diff --git a/packages/core/src/__tests__/insight-store.test.ts b/packages/core/src/__tests__/insight-store.test.ts
index 192f454f17..5fedaee5fa 100644
--- a/packages/core/src/__tests__/insight-store.test.ts
+++ b/packages/core/src/__tests__/insight-store.test.ts
@@ -1000,7 +1000,7 @@ describe("Migration: pre-33 DB upgrade", () => {
       // Step 1: Create a fresh database at v33 (runs all migrations up to 33)
       const db1 = createDatabase(legacyDir);
       db1.init();
-      expect(db1.getSchemaVersion()).toBe(105);
+      expect(db1.getSchemaVersion()).toBe(106);
       db1.close();
 
       // Step 2: Manually downgrade to version 32 and drop insight tables
@@ -1035,7 +1035,7 @@ describe("Migration: pre-33 DB upgrade", () => {
       expect(tableNamesBefore).not.toContain("project_insight_runs");
       // Now run init — this triggers the v32→v33 migration
       db3.init();
-      expect(db3.getSchemaVersion()).toBe(105);
+      expect(db3.getSchemaVersion()).toBe(106);
 
       // Step 4: Verify insight tables exist after migration
       const tablesAfter = db3.prepare(
@@ -1066,12 +1066,12 @@ describe("Migration: pre-33 DB upgrade", () => {
     try {
       const db1 = createDatabase(testDir);
       db1.init();
-      expect(db1.getSchemaVersion()).toBe(105);
+      expect(db1.getSchemaVersion()).toBe(106);
       db1.close();
 
       const db2 = createDatabase(testDir);
       expect(() => db2.init()).not.toThrow();
-      expect(db2.getSchemaVersion()).toBe(105);
+      expect(db2.getSchemaVersion()).toBe(106);
       db2.close();
     } finally {
       rmSync(testDir, { recursive: true, force: true });
@@ -1085,7 +1085,7 @@ describe("Migration: pre-33 DB upgrade", () => {
       // Step 1: Create a fresh DB and run migrations
       const db1 = createDatabase(compatDir);
       db1.init();
-      expect(db1.getSchemaVersion()).toBe(105);
+      expect(db1.getSchemaVersion()).toBe(106);
 
       // Step 2: Strip lifecycle and cancelledAt columns by recreating the
       // table without them. This simulates a DB that was created before the
diff --git a/packages/core/src/__tests__/merge-request-record.test.ts b/packages/core/src/__tests__/merge-request-record.test.ts
index ca23e17058..d550666ccd 100644
--- a/packages/core/src/__tests__/merge-request-record.test.ts
+++ b/packages/core/src/__tests__/merge-request-record.test.ts
@@ -38,7 +38,7 @@ describe("TaskStore merge request record + completion handoff marker", () => {
       .all() as Array<{ name: string }>;
 
     expect(tableRows).toEqual([{ name: "completion_handoff_markers" }, { name: "merge_requests" }]);
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
   });
 
   it("upserts merge request records", async () => {
diff --git a/packages/core/src/__tests__/mission-store.test.ts b/packages/core/src/__tests__/mission-store.test.ts
index 8909b33c5f..3b6148204b 100644
--- a/packages/core/src/__tests__/mission-store.test.ts
+++ b/packages/core/src/__tests__/mission-store.test.ts
@@ -3746,7 +3746,7 @@ describe("MissionStore", () => {
 
   describe("Loop State & Validator Run Schema (v31)", () => {
     it("schema version is 101 after migration", () => {
-      expect(db.getSchemaVersion()).toBe(105);
+      expect(db.getSchemaVersion()).toBe(106);
     });
 
     it("mission_features table has loop state columns", () => {
diff --git a/packages/core/src/__tests__/run-audit.test.ts b/packages/core/src/__tests__/run-audit.test.ts
index 855c54b1c2..b483a25daa 100644
--- a/packages/core/src/__tests__/run-audit.test.ts
+++ b/packages/core/src/__tests__/run-audit.test.ts
@@ -584,7 +584,7 @@ describe("Run Audit", () => {
     });
 
     it("schema version is bumped to 40", () => {
-      expect(db.getSchemaVersion()).toBe(105);
+      expect(db.getSchemaVersion()).toBe(106);
     });
   });
 });
diff --git a/packages/core/src/__tests__/store-merge-queue.test.ts b/packages/core/src/__tests__/store-merge-queue.test.ts
index 646438bbec..97fad4deca 100644
--- a/packages/core/src/__tests__/store-merge-queue.test.ts
+++ b/packages/core/src/__tests__/store-merge-queue.test.ts
@@ -60,7 +60,7 @@ describe("TaskStore merge queue", () => {
       expect.arrayContaining(["idx_mergeQueue_lease_ready", "idx_mergeQueue_leaseExpiresAt"]),
     );
 
-    expect(store.getDatabase().getSchemaVersion()).toBe(105);
+    expect(store.getDatabase().getSchemaVersion()).toBe(106);
   });
 
   it("migrates a legacy v88 database and preserves task rows", async () => {
diff --git a/packages/core/src/__tests__/task-documents.test.ts b/packages/core/src/__tests__/task-documents.test.ts
index 6e442a3f1b..9fec8d9c06 100644
--- a/packages/core/src/__tests__/task-documents.test.ts
+++ b/packages/core/src/__tests__/task-documents.test.ts
@@ -51,7 +51,7 @@ describe("TaskStore task documents", () => {
 
     expect(tableNames.has("task_documents")).toBe(true);
     expect(tableNames.has("task_document_revisions")).toBe(true);
-    expect(db.getSchemaVersion()).toBe(105);
+    expect(db.getSchemaVersion()).toBe(106);
 
     const index = db
       .prepare(
diff --git a/packages/core/src/__tests__/transition-types.test.ts b/packages/core/src/__tests__/transition-types.test.ts
new file mode 100644
index 0000000000..6eaad0cc76
--- /dev/null
+++ b/packages/core/src/__tests__/transition-types.test.ts
@@ -0,0 +1,281 @@
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { mkdtempSync } from "node:fs";
+import { rm } from "node:fs/promises";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { Database, SCHEMA_VERSION } from "../db.js";
+import {
+  TRANSITION_REJECTION_CODES,
+  type TransitionRejectionCode,
+  deserializeTransitionPending,
+  deserializeTransitionRejection,
+  makeTransitionPending,
+  makeTransitionRejection,
+  serializeTransitionPending,
+  serializeTransitionRejection,
+  transitionOk,
+  transitionRejected,
+} from "../transition-types.js";
+import {
+  clearTransitionPending,
+  reconcileHooksRemaining,
+  readTransitionPending,
+  writeTransitionPending,
+} from "../transition-pending.js";
+
+function makeTmpDir(): string {
+  return mkdtempSync(join(tmpdir(), "kb-transition-types-"));
+}
+
+describe("TransitionRejection (de)serialization across the API boundary", () => {
+  it("round-trips every rejection code", () => {
+    for (const code of TRANSITION_REJECTION_CODES) {
+      const rejection = makeTransitionRejection(code, `transition.reject.${code}`, code === "capacity-exhausted");
+      const wire = serializeTransitionRejection(rejection);
+      // Wire form is plain JSON — no class instances survive the boundary.
+      expect(typeof wire).toBe("string");
+      const parsedRaw = JSON.parse(wire) as Record<string, unknown>;
+      expect(parsedRaw.code).toBe(code);
+      const back = deserializeTransitionRejection(wire);
+      expect(back).toEqual(rejection);
+    }
+  });
+
+  it("round-trips the optional detail field and omits it when absent", () => {
+    const withDetail = makeTransitionRejection("guard-rejected", "k", false, "guard X said no");
+    expect(deserializeTransitionRejection(serializeTransitionRejection(withDetail))).toEqual(withDetail);
+
+    const withoutDetail = makeTransitionRejection("unknown-column", "k", false);
+    expect("detail" in withoutDetail).toBe(false);
+    const wire = serializeTransitionRejection(withoutDetail);
+    expect(JSON.parse(wire)).not.toHaveProperty("detail");
+    expect(deserializeTransitionRejection(wire)).toEqual(withoutDetail);
+  });
+
+  it("rejects malformed / structurally invalid payloads with null (never throws)", () => {
+    expect(deserializeTransitionRejection("not json{{")).toBeNull();
+    expect(deserializeTransitionRejection("null")).toBeNull();
+    expect(deserializeTransitionRejection("42")).toBeNull();
+    expect(deserializeTransitionRejection(JSON.stringify({ code: "not-a-code", messageKey: "k", retryable: true }))).toBeNull();
+    expect(deserializeTransitionRejection(JSON.stringify({ code: "guard-rejected", retryable: true }))).toBeNull();
+    expect(deserializeTransitionRejection(JSON.stringify({ code: "guard-rejected", messageKey: "k", retryable: "yes" }))).toBeNull();
+    expect(deserializeTransitionRejection(JSON.stringify({ code: "guard-rejected", messageKey: "k", retryable: true, detail: 7 }))).toBeNull();
+  });
+
+  it("builds discriminated TransitionResult values", () => {
+    const ok = transitionOk("in-review");
+    expect(ok).toEqual({ ok: true, toColumn: "in-review" });
+
+    const rejection = makeTransitionRejection("merge-blocked", "transition.merge-blocked", true);
+    const rejected = transitionRejected(rejection);
+    expect(rejected).toEqual({ ok: false, rejection });
+  });
+
+  it("exposes the full, exhaustive code set", () => {
+    const expected: TransitionRejectionCode[] = [
+      "guard-rejected",
+      "capacity-exhausted",
+      "unknown-column",
+      "workflow-mismatch",
+      "merge-blocked",
+    ];
+    expect([...TRANSITION_REJECTION_CODES].sort()).toEqual([...expected].sort());
+  });
+});
+
+describe("TransitionPending (de)serialization", () => {
+  it("round-trips a marker including hooksRemaining order and startedAt", () => {
+    const marker = makeTransitionPending("in-progress", ["timing:onEnter", "abort-on-exit:onExit"], 1_700_000_000_000);
+    const wire = serializeTransitionPending(marker);
+    expect(deserializeTransitionPending(wire)).toEqual(marker);
+  });
+
+  it("copies hooksRemaining so the marker does not alias caller state", () => {
+    const hooks = ["a", "b"];
+    const marker = makeTransitionPending("todo", hooks);
+    hooks.push("c");
+    expect(marker.hooksRemaining).toEqual(["a", "b"]);
+  });
+
+  it("drops non-string hook entries defensively and rejects malformed markers", () => {
+    expect(deserializeTransitionPending("garbage")).toBeNull();
+    expect(deserializeTransitionPending(JSON.stringify({ toColumn: "x", startedAt: 1 }))).toBeNull();
+    expect(deserializeTransitionPending(JSON.stringify({ toColumn: "x", hooksRemaining: [], startedAt: "soon" }))).toBeNull();
+    const recovered = deserializeTransitionPending(
+      JSON.stringify({ toColumn: "x", hooksRemaining: ["keep", 5, null, "also"], startedAt: 10 }),
+    );
+    expect(recovered).toEqual({ toColumn: "x", hooksRemaining: ["keep", "also"], startedAt: 10 });
+  });
+});
+
+describe("reconcileHooksRemaining (missing-plugin-hook, U3-level)", () => {
+  it("keeps known hooks and drops unknown ones with one audit warning each", () => {
+    const known = new Set(["builtin:timing", "builtin:abort"]);
+    const result = reconcileHooksRemaining(["builtin:timing", "plugin:gone", "builtin:abort", "plugin:also-gone"], known);
+    expect(result.hooksRemaining).toEqual(["builtin:timing", "builtin:abort"]);
+    expect(result.warnings).toHaveLength(2);
+    expect(result.warnings[0]).toContain("plugin:gone");
+    expect(result.warnings[1]).toContain("plugin:also-gone");
+  });
+
+  it("returns no warnings when every hook is known", () => {
+    const result = reconcileHooksRemaining(["a"], new Set(["a", "b"]));
+    expect(result).toEqual({ hooksRemaining: ["a"], warnings: [] });
+  });
+});
+
+describe("transitionPending marker lifecycle (helper-level, U3)", () => {
+  let tmpDir: string;
+  let fusionDir: string;
+  let db: Database;
+
+  beforeEach(() => {
+    tmpDir = makeTmpDir();
+    fusionDir = join(tmpDir, ".fusion");
+    db = new Database(fusionDir);
+    db.init();
+    db.exec(
+      `INSERT INTO tasks (id, description, "column", createdAt, updatedAt) VALUES ('FN-1', 'task', 'todo', '2025-01-01T00:00:00.000Z', '2025-01-01T00:00:00.000Z')`,
+    );
+  });
+
+  afterEach(async () => {
+    try {
+      db.close();
+    } catch {
+      // already closed
+    }
+    await rm(tmpDir, { recursive: true, force: true });
+  });
+
+  it("set with a move, then cleared after hooks complete", () => {
+    expect(readTransitionPending(db, "FN-1")).toBeNull();
+
+    // Simulate the in-txn write that accompanies a column change (U4 wires this):
+    // the column change and the marker write land in one transaction.
+    db.exec("BEGIN");
+    db.prepare(`UPDATE tasks SET "column" = ? WHERE id = ?`).run("in-progress", "FN-1");
+    writeTransitionPending(db, "FN-1", makeTransitionPending("in-progress", ["timing:onEnter"], 1234));
+    db.exec("COMMIT");
+
+    const after = readTransitionPending(db, "FN-1");
+    expect(after).toEqual({ toColumn: "in-progress", hooksRemaining: ["timing:onEnter"], startedAt: 1234 });
+    const movedRow = db.prepare(`SELECT "column" AS col FROM tasks WHERE id = ?`).get("FN-1") as { col: string };
+    expect(movedRow.col).toBe("in-progress");
+
+    // Post-commit hooks ran -> clear.
+    clearTransitionPending(db, "FN-1");
+    expect(readTransitionPending(db, "FN-1")).toBeNull();
+  });
+
+  it("survives a simulated crash: marker recoverable with hooksRemaining intact", () => {
+    writeTransitionPending(db, "FN-1", makeTransitionPending("in-review", ["merge:onEnter", "stall:onEnter"], 999));
+    db.close();
+
+    // Re-open as a fresh handle (the post-commit hook runner never ran -> crash).
+    const reopened = new Database(fusionDir);
+    reopened.init();
+    const recovered = readTransitionPending(reopened, "FN-1");
+    expect(recovered).toEqual({ toColumn: "in-review", hooksRemaining: ["merge:onEnter", "stall:onEnter"], startedAt: 999 });
+    reopened.close();
+    db = new Database(fusionDir);
+    db.init();
+  });
+
+  it("reads back exclusively from the SQLite row (authoritative store, ADR-0001)", () => {
+    // The helper only ever consults the SQLite tasks row; there is no task.json
+    // read path. Writing the marker and reading it through a brand-new handle
+    // proves SQLite is the single source of truth.
+    writeTransitionPending(db, "FN-1", makeTransitionPending("done", ["complete:onEnter"], 5));
+    db.close();
+    const fresh = new Database(fusionDir);
+    fresh.init();
+    expect(readTransitionPending(fresh, "FN-1")).toEqual({
+      toColumn: "done",
+      hooksRemaining: ["complete:onEnter"],
+      startedAt: 5,
+    });
+    fresh.close();
+    db = new Database(fusionDir);
+    db.init();
+  });
+
+  it("returns undefined for a missing task and null for a corrupt marker", () => {
+    expect(readTransitionPending(db, "FN-nonexistent")).toBeUndefined();
+    db.prepare(`UPDATE tasks SET transitionPending = ? WHERE id = ?`).run("not json{{", "FN-1");
+    expect(readTransitionPending(db, "FN-1")).toBeNull();
+  });
+});
+
+describe("tasks.transitionPending migration (106)", () => {
+  let tmpDir: string;
+  let fusionDir: string;
+
+  beforeEach(() => {
+    tmpDir = makeTmpDir();
+    fusionDir = join(tmpDir, ".fusion");
+  });
+
+  afterEach(async () => {
+    await rm(tmpDir, { recursive: true, force: true });
+  });
+
+  it("adds the column when migrating a pre-106 tasks table, leaving existing rows NULL", () => {
+    const db = new Database(fusionDir);
+    db.exec("CREATE TABLE IF NOT EXISTS __meta (key TEXT PRIMARY KEY, value TEXT)");
+    db.exec(`
+      CREATE TABLE IF NOT EXISTS tasks (
+        id TEXT PRIMARY KEY,
+        description TEXT NOT NULL,
+        "column" TEXT NOT NULL,
+        createdAt TEXT NOT NULL,
+        updatedAt TEXT NOT NULL
+      )
+    `);
+    db.exec("INSERT INTO __meta (key, value) VALUES ('schemaVersion', '105')");
+    db.exec("INSERT INTO __meta (key, value) VALUES ('lastModified', '1000')");
+    db.exec(
+      `INSERT INTO tasks (id, description, "column", createdAt, updatedAt) VALUES ('FN-legacy', 'legacy', 'todo', '2025-01-01T00:00:00.000Z', '2025-01-01T00:00:00.000Z')`,
+    );
+
+    db.init();
+
+    const columns = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
+    expect(columns.map((c) => c.name)).toContain("transitionPending");
+
+    const row = db.prepare("SELECT transitionPending FROM tasks WHERE id = 'FN-legacy'").get() as {
+      transitionPending: string | null;
+    };
+    expect(row.transitionPending).toBeNull();
+    expect(db.getSchemaVersion()).toBe(SCHEMA_VERSION);
+
+    db.close();
+  });
+
+  it("is idempotent: running init twice does not error and stays at the current version", () => {
+    const db = new Database(fusionDir);
+    db.init();
+    expect(db.getSchemaVersion()).toBe(SCHEMA_VERSION);
+    // Second init on the same DB is a no-op (version already current).
+    expect(() => db.init()).not.toThrow();
+    expect(db.getSchemaVersion()).toBe(SCHEMA_VERSION);
+    const columns = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
+    expect(columns.filter((c) => c.name === "transitionPending")).toHaveLength(1);
+    db.close();
+
+    // Re-open + init a third time on the persisted DB.
+    const reopened = new Database(fusionDir);
+    expect(() => reopened.init()).not.toThrow();
+    expect(reopened.getSchemaVersion()).toBe(SCHEMA_VERSION);
+    reopened.close();
+  });
+
+  it("is a no-op on a fresh DB: column present from the base CREATE TABLE", () => {
+    const db = new Database(fusionDir);
+    db.init();
+    const columns = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
+    expect(columns.map((c) => c.name)).toContain("transitionPending");
+    expect(db.getSchemaVersion()).toBe(SCHEMA_VERSION);
+    db.close();
+  });
+});
diff --git a/packages/core/src/db.ts b/packages/core/src/db.ts
index f9ea699ad4..e23e1f543b 100644
--- a/packages/core/src/db.ts
+++ b/packages/core/src/db.ts
@@ -149,7 +149,7 @@ export function probeFts5(db: DatabaseSync): boolean {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 105;
+const SCHEMA_VERSION = 106;
 
 export { SCHEMA_VERSION };
 
@@ -322,7 +322,8 @@ CREATE TABLE IF NOT EXISTS tasks (
   checkoutLeaseRenewedAt TEXT,
   checkoutLeaseEpoch INTEGER DEFAULT 0,
   deletedAt TEXT,
-  allowResurrection INTEGER DEFAULT 0
+  allowResurrection INTEGER DEFAULT 0,
+  transitionPending TEXT
 );
 
 -- Config table (single row with project settings)
@@ -4181,6 +4182,17 @@ export class Database {
       });
     }
 
+    // Migration 106: Crash-safe transition marker (workflow-columns U3). Stores
+    // JSON {toColumn, hooksRemaining, startedAt} written in the same txn as a
+    // column change; recovery re-runs the remaining idempotent post-commit hooks
+    // and clears it. Additive-only, nullable, no backfill — existing rows have
+    // no in-flight transition.
+    if (version < 106) {
+      this.applyMigration(106, () => {
+        this.addColumnIfMissing("tasks", "transitionPending", "TEXT");
+      });
+    }
+
   }
 
   /**
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index d76c50978c..da4b6ccec2 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -65,6 +65,71 @@ export type {
   WorkflowJoinBranchFailure,
 } from "./workflow-ir-types.js";
 export { BUILTIN_CODING_WORKFLOW_IR } from "./builtin-coding-workflow-ir.js";
+
+// ── Trait model (U2) ─────────────────────────────────────────────────
+export type {
+  TraitDefinition,
+  TraitFlags,
+  TraitConfigSchema,
+  TraitConfigField,
+  TraitHookDescriptors,
+  TraitHookKind,
+  TraitHookImpl,
+  RestrictedTraitFlag,
+} from "./trait-types.js";
+export { RESTRICTED_TRAIT_FLAGS, traitHookKey } from "./trait-types.js";
+export {
+  TraitRegistry,
+  TraitRegistrationError,
+  getTraitRegistry,
+  getTrait,
+  listTraits,
+  resolveColumnFlags,
+  validateColumnTraits,
+  registerTraitHookImpl,
+  __resetTraitRegistryForTests,
+} from "./trait-registry.js";
+export type {
+  TraitRegistrationReason,
+  TraitViolation,
+  TraitViolationCode,
+  TraitViolationSeverity,
+  TraitAuditWarning,
+} from "./trait-registry.js";
+export {
+  BUILTIN_TRAIT_IDS,
+  BUILTIN_TRAIT_DEFINITIONS,
+  registerBuiltinTraits,
+} from "./builtin-traits.js";
+export type { BuiltinTraitId } from "./builtin-traits.js";
+// ── Typed transition contract + crash-safe marker (U3) ───────────────
+export type {
+  TransitionRejection,
+  TransitionRejectionCode,
+  TransitionResult,
+  TransitionPending,
+} from "./transition-types.js";
+export {
+  TRANSITION_REJECTION_CODES,
+  makeTransitionRejection,
+  makeTransitionPending,
+  transitionOk,
+  transitionRejected,
+  serializeTransitionRejection,
+  deserializeTransitionRejection,
+  serializeTransitionPending,
+  deserializeTransitionPending,
+} from "./transition-types.js";
+export type {
+  TransitionPendingDbHandle,
+  ReconcileHooksResult,
+} from "./transition-pending.js";
+export {
+  readTransitionPending,
+  writeTransitionPending,
+  clearTransitionPending,
+  reconcileHooksRemaining,
+} from "./transition-pending.js";
 export type {
   WorkflowDefinition,
   WorkflowDefinitionInput,
diff --git a/packages/core/src/transition-pending.ts b/packages/core/src/transition-pending.ts
new file mode 100644
index 0000000000..6f09f324ff
--- /dev/null
+++ b/packages/core/src/transition-pending.ts
@@ -0,0 +1,115 @@
+/**
+ * Store-side read/write helpers for the crash-safe `tasks.transitionPending`
+ * marker (U3).
+ *
+ * These operate on a minimal db handle (anything exposing a `prepare` that
+ * returns a statement with `.get`/`.run`) so they can be unit-tested against a
+ * raw {@link import("./db.js").Database} without dragging in `store.ts`. U4 owns
+ * wiring these into `moveTaskInternal`'s transaction and the recovery sweep;
+ * this module is the clean seam they will call.
+ *
+ * The marker is written in the same transaction as the column change (KTD-2) and
+ * cleared once post-commit hooks complete. Recovery reads it back exclusively
+ * from SQLite (the authoritative store per ADR-0001).
+ */
+
+import {
+  type TransitionPending,
+  deserializeTransitionPending,
+  serializeTransitionPending,
+} from "./transition-types.js";
+
+/** Minimal statement surface the helpers need (subset of node:sqlite's StatementSync). */
+interface MarkerStatement {
+  get(...params: unknown[]): unknown;
+  run(...params: unknown[]): unknown;
+}
+
+/** Minimal db handle: just enough to prepare statements. Satisfied by `Database`. */
+export interface TransitionPendingDbHandle {
+  prepare(sql: string): MarkerStatement;
+}
+
+/**
+ * Read the pending marker for a task. Returns `null` when the column is NULL,
+ * empty, or holds malformed JSON (a corrupt marker must never throw on a
+ * recovery path — it degrades to "no pending work" and the row is treated as
+ * settled). Returns `undefined` only when the task row does not exist.
+ */
+export function readTransitionPending(
+  db: TransitionPendingDbHandle,
+  taskId: string,
+): TransitionPending | null | undefined {
+  const row = db
+    .prepare(`SELECT transitionPending FROM tasks WHERE id = ?`)
+    .get(taskId) as { transitionPending: string | null } | undefined;
+  if (row === undefined) return undefined;
+  if (row.transitionPending == null || row.transitionPending === "") return null;
+  return deserializeTransitionPending(row.transitionPending);
+}
+
+/**
+ * Write (set or replace) the pending marker for a task. Intended to run inside
+ * the same transaction as the column change (U4). Stores the JSON-serialized
+ * marker into `tasks.transitionPending`.
+ */
+export function writeTransitionPending(
+  db: TransitionPendingDbHandle,
+  taskId: string,
+  pending: TransitionPending,
+): void {
+  db.prepare(`UPDATE tasks SET transitionPending = ? WHERE id = ?`).run(
+    serializeTransitionPending(pending),
+    taskId,
+  );
+}
+
+/**
+ * Clear the pending marker for a task (sets the column to NULL). Called once all
+ * post-commit hooks for the transition have completed.
+ */
+export function clearTransitionPending(db: TransitionPendingDbHandle, taskId: string): void {
+  db.prepare(`UPDATE tasks SET transitionPending = NULL WHERE id = ?`).run(taskId);
+}
+
+/** Result of reconciling a marker's `hooksRemaining` against the known hook set. */
+export interface ReconcileHooksResult {
+  /** Hooks that survived: still registered/known and owed execution. */
+  hooksRemaining: string[];
+  /**
+   * Audit warnings for each dropped hook entry — e.g. a hook belonging to a
+   * now-uninstalled plugin. One human-readable message per dropped entry so the
+   * recovery sweep can emit a degraded-hook audit event and complete the marker
+   * rather than leaving the card stuck waiting for a missing handler.
+   */
+  warnings: string[];
+}
+
+/**
+ * Reconcile a marker's `hooksRemaining` against the set of currently-known hook
+ * IDs. Entries no longer present (e.g. a plugin hook removed by uninstall) are
+ * dropped and surfaced as audit warnings. Pure — no DB access — so U4/U8 can
+ * call it in or out of a transaction.
+ *
+ * This covers the U3-level slice of the "missing-plugin-hook" scenario: the
+ * type/helper guarantees a dangling hook entry resolves to a dropped entry plus
+ * a warning, never an indefinitely-stuck marker. The actual recovery wiring is
+ * U4/U8.
+ */
+export function reconcileHooksRemaining(
+  hooksRemaining: readonly string[],
+  knownHookIds: ReadonlySet<string>,
+): ReconcileHooksResult {
+  const surviving: string[] = [];
+  const warnings: string[] = [];
+  for (const hookId of hooksRemaining) {
+    if (knownHookIds.has(hookId)) {
+      surviving.push(hookId);
+    } else {
+      warnings.push(
+        `Dropping unknown transition hook "${hookId}" from transitionPending marker (handler not registered; likely an uninstalled plugin)`,
+      );
+    }
+  }
+  return { hooksRemaining: surviving, warnings };
+}
diff --git a/packages/core/src/transition-types.ts b/packages/core/src/transition-types.ts
new file mode 100644
index 0000000000..728445afee
--- /dev/null
+++ b/packages/core/src/transition-types.ts
@@ -0,0 +1,189 @@
+/**
+ * Typed transition contract (U3).
+ *
+ * `moveTaskInternal` (the single transition authority, KTD-3/R13) stops throwing
+ * bare strings on a rejected move and instead returns a typed {@link TransitionResult}.
+ * The rejection shape is shared verbatim across surfaces — the dashboard drop
+ * handler, the CLI, the HTTP move endpoint, and the recovery sweep — so every
+ * caller speaks one rejection contract. Because the rejection crosses the HTTP
+ * API boundary, the type is intentionally a flat, JSON-safe object (no class
+ * instances, no functions, no `undefined`-only fields that would survive a JSON
+ * round-trip differently than declared) and ships with explicit
+ * (de)serialization helpers below.
+ *
+ * The {@link TransitionPending} marker is the crash-safe hook protocol (KTD-2/KTD-9):
+ * it is written in the same SQLite transaction as the column change and records
+ * which post-commit, idempotent enter/exit hooks still owe execution. A crash
+ * mid-transition leaves the marker behind; the recovery sweep re-reads it from
+ * SQLite (the authoritative store per ADR-0001 — `task.json` is a stale follower
+ * across a crash) and re-runs the remaining idempotent hooks. The marker, like
+ * the rejection, crosses no class boundary and round-trips cleanly through JSON.
+ */
+
+/**
+ * Reason codes for a rejected transition. Stable string literals — they are
+ * persisted in audit and matched by surfaces to choose user-facing copy, so
+ * they must not change without a migration of the consumers.
+ */
+export type TransitionRejectionCode =
+  | "guard-rejected"
+  | "capacity-exhausted"
+  | "unknown-column"
+  | "workflow-mismatch"
+  | "merge-blocked";
+
+/** The full, immutable set of rejection codes (handy for exhaustive validation). */
+export const TRANSITION_REJECTION_CODES: readonly TransitionRejectionCode[] = [
+  "guard-rejected",
+  "capacity-exhausted",
+  "unknown-column",
+  "workflow-mismatch",
+  "merge-blocked",
+] as const;
+
+/**
+ * A typed transition rejection. Flat and JSON-safe by construction.
+ *
+ * - `code` — machine-stable {@link TransitionRejectionCode}.
+ * - `messageKey` — i18n key the surface resolves to user-facing copy (never a
+ *   pre-translated string; translation is the surface's job).
+ * - `retryable` — whether re-issuing the same move could succeed later (e.g. a
+ *   capacity exhaustion frees up) versus a structural rejection that will not
+ *   (e.g. unknown column).
+ * - `detail` — optional, non-localized diagnostic context for audit/logs only.
+ */
+export interface TransitionRejection {
+  code: TransitionRejectionCode;
+  messageKey: string;
+  retryable: boolean;
+  detail?: string;
+}
+
+/**
+ * Result of an attempted transition. Discriminated on `ok` so callers branch
+ * exhaustively. The success arm carries the resolved destination column so the
+ * caller need not re-read it.
+ */
+export type TransitionResult =
+  | { ok: true; toColumn: string }
+  | { ok: false; rejection: TransitionRejection };
+
+/**
+ * Crash-safe marker persisted alongside the column change. `hooksRemaining`
+ * holds the IDs of post-commit enter/exit hooks that have not yet completed;
+ * recovery re-runs exactly these (idempotently) and clears the marker when the
+ * list empties. `startedAt` is an epoch-millis timestamp used for stall/age
+ * diagnostics and ordering during recovery.
+ */
+export interface TransitionPending {
+  toColumn: string;
+  hooksRemaining: string[];
+  startedAt: number;
+}
+
+// ---------------------------------------------------------------------------
+// Helper constructors
+// ---------------------------------------------------------------------------
+
+/**
+ * Construct a {@link TransitionRejection}. `detail` is omitted from the object
+ * when not supplied so the serialized shape stays minimal and stable.
+ */
+export function makeTransitionRejection(
+  code: TransitionRejectionCode,
+  messageKey: string,
+  retryable: boolean,
+  detail?: string,
+): TransitionRejection {
+  const rejection: TransitionRejection = { code, messageKey, retryable };
+  if (detail !== undefined) {
+    rejection.detail = detail;
+  }
+  return rejection;
+}
+
+/** Construct a successful {@link TransitionResult}. */
+export function transitionOk(toColumn: string): TransitionResult {
+  return { ok: true, toColumn };
+}
+
+/** Construct a rejected {@link TransitionResult} from a rejection. */
+export function transitionRejected(rejection: TransitionRejection): TransitionResult {
+  return { ok: false, rejection };
+}
+
+/**
+ * Construct a {@link TransitionPending} marker. `startedAt` defaults to now so
+ * the common call site (`moveTaskInternal` writing the marker in-txn) stays
+ * terse; callers reconstructing a marker from a stored value pass it explicitly.
+ * The `hooksRemaining` array is copied so the marker does not alias caller state.
+ */
+export function makeTransitionPending(
+  toColumn: string,
+  hooksRemaining: string[],
+  startedAt: number = Date.now(),
+): TransitionPending {
+  return { toColumn, hooksRemaining: [...hooksRemaining], startedAt };
+}
+
+// ---------------------------------------------------------------------------
+// (De)serialization — JSON-safe round-trip across the API boundary
+// ---------------------------------------------------------------------------
+
+function isTransitionRejectionCode(value: unknown): value is TransitionRejectionCode {
+  return typeof value === "string" && (TRANSITION_REJECTION_CODES as readonly string[]).includes(value);
+}
+
+/** Serialize a rejection to a JSON string for transport/persistence. */
+export function serializeTransitionRejection(rejection: TransitionRejection): string {
+  return JSON.stringify(rejection);
+}
+
+/**
+ * Parse a rejection from a JSON string produced by
+ * {@link serializeTransitionRejection}. Returns `null` for malformed or
+ * structurally invalid input rather than throwing, so a corrupt audit payload
+ * can never crash a recovery path.
+ */
+export function deserializeTransitionRejection(json: string): TransitionRejection | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(json);
+  } catch {
+    return null;
+  }
+  if (typeof parsed !== "object" || parsed === null) return null;
+  const obj = parsed as Record<string, unknown>;
+  if (!isTransitionRejectionCode(obj.code)) return null;
+  if (typeof obj.messageKey !== "string") return null;
+  if (typeof obj.retryable !== "boolean") return null;
+  if (obj.detail !== undefined && typeof obj.detail !== "string") return null;
+  return makeTransitionRejection(obj.code, obj.messageKey, obj.retryable, obj.detail as string | undefined);
+}
+
+/** Serialize a pending marker to a JSON string for the `tasks.transitionPending` column. */
+export function serializeTransitionPending(pending: TransitionPending): string {
+  return JSON.stringify(pending);
+}
+
+/**
+ * Parse a pending marker from the JSON stored in `tasks.transitionPending`.
+ * Returns `null` for malformed/invalid input. Non-string entries in
+ * `hooksRemaining` are dropped defensively (a corrupt array element must not
+ * strand the card); the structural shape is otherwise required.
+ */
+export function deserializeTransitionPending(json: string): TransitionPending | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(json);
+  } catch {
+    return null;
+  }
+  if (typeof parsed !== "object" || parsed === null) return null;
+  const obj = parsed as Record<string, unknown>;
+  if (typeof obj.toColumn !== "string") return null;
+  if (!Array.isArray(obj.hooksRemaining)) return null;
+  if (typeof obj.startedAt !== "number" || !Number.isFinite(obj.startedAt)) return null;
+  const hooksRemaining = obj.hooksRemaining.filter((h): h is string => typeof h === "string");
+  return { toColumn: obj.toColumn, hooksRemaining, startedAt: obj.startedAt };
+}