feat(FN-3878): add restore-merge-sha script for commitSha restoration

Adds a new merge commitSha restoration script (`scripts/restore-merge-sha-fn-3878.mjs`) with test coverage to handle a specific FN-3878 merge case, along with a changeset for publishing and a minor plugin vitest config governance alignment across three plugins.

Fusion-Task-Id: FN-3878

scripts/__tests__/restore-merge-sha-fn-3878.test.mjs

@@ -0,0 +1,152 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { runRestoration } from "../restore-merge-sha-fn-3878.mjs";
+
+function createStore(task) {
+  const state = { ...task, mergeDetails: { ...(task.mergeDetails ?? {}) } };
+  const calls = { updateTask: 0, logEntry: 0 };
+  return {
+    calls,
+    state,
+    async getTask(id) {
+      assert.equal(id, state.id);
+      return { ...state, mergeDetails: { ...state.mergeDetails } };
+    },
+    async updateTask(id, updates) {
+      assert.equal(id, state.id);
+      calls.updateTask += 1;
+      state.mergeDetails = { ...updates.mergeDetails };
+      return { ...state };
+    },
+    async logEntry(id, action, outcome) {
+      assert.equal(id, state.id);
+      assert.equal(action, "FN-3878 restore commitSha");
+      assert.match(outcome, /→/);
+      calls.logEntry += 1;
+      return { ...state };
+    },
+  };
+}
+
+function createGit({ ancestor = true, owned = true } = {}) {
+  return {
+    isAncestorOfMain() {
+      return ancestor;
+    },
+    getCommitSubject() {
+      return owned ? "fix(FN-3794): canonical" : "fix: unrelated";
+    },
+    getCommitBody() {
+      return owned ? "Body\n\nFusion-Task-Id: FN-3794" : "Body without trailer";
+    },
+    getCommitAuthorDateIso() {
+      return "2026-05-09T00:50:47-07:00";
+    },
+    getShortstat() {
+      return { filesChanged: 3, insertions: 20, deletions: 5 };
+    },
+  };
+}
+
+const restoration = [{ id: "FN-3794", canonicalSha: "7d20a348d82320bc57310169aaa2d3b3f0d5a946" }];
+
+function baseTask() {
+  return {
+    id: "FN-3794",
+    column: "done",
+    mergeDetails: {
+      commitSha: "oldoldoldoldoldoldoldoldoldoldoldoldoldoldold1",
+      mergeConfirmed: true,
+      branch: "ignored",
+      resolutionStrategy: "ai",
+      resolutionMethod: "ai",
+      attemptsMade: 2,
+      autoResolvedCount: 4,
+    },
+  };
+}
+
+test("FN-3878: dry-run by default — no DB writes", async () => {
+  const store = createStore(baseTask());
+  const result = await runRestoration({
+    store,
+    git: createGit(),
+    restorations: restoration,
+    dryRun: true,
+  });
+
+  assert.equal(result.hadValidationErrors, false);
+  assert.equal(store.calls.updateTask, 0);
+  assert.equal(store.calls.logEntry, 0);
+  assert.equal(store.state.mergeDetails.commitSha, "oldoldoldoldoldoldoldoldoldoldoldoldoldoldold1");
+  assert.equal(result.results[0].action, "updated");
+  assert.equal(result.results[0].reason, "dry-run");
+});
+
+test("FN-3878: --apply rewrites commitSha and preserves mergeConfirmed + strategy metadata", async () => {
+  const store = createStore(baseTask());
+  await runRestoration({
+    store,
+    git: createGit(),
+    restorations: restoration,
+    dryRun: false,
+  });
+
+  assert.equal(store.calls.updateTask, 1);
+  assert.equal(store.calls.logEntry, 1);
+  assert.equal(store.state.mergeDetails.commitSha, restoration[0].canonicalSha);
+  assert.equal(store.state.mergeDetails.mergeCommitMessage, "fix(FN-3794): canonical");
+  assert.equal(store.state.mergeDetails.filesChanged, 3);
+  assert.equal(store.state.mergeDetails.insertions, 20);
+  assert.equal(store.state.mergeDetails.deletions, 5);
+  assert.equal(store.state.mergeDetails.mergedAt, "2026-05-09T00:50:47-07:00");
+  assert.equal(store.state.mergeDetails.mergeConfirmed, true);
+  assert.equal(store.state.mergeDetails.resolutionStrategy, "ai");
+  assert.equal(store.state.mergeDetails.resolutionMethod, "ai");
+  assert.equal(store.state.mergeDetails.attemptsMade, 2);
+  assert.equal(store.state.mergeDetails.autoResolvedCount, 4);
+});
+
+test("FN-3878: idempotent — re-running with --apply is a no-op", async () => {
+  const store = createStore(baseTask());
+  await runRestoration({ store, git: createGit(), restorations: restoration, dryRun: false });
+  const second = await runRestoration({ store, git: createGit(), restorations: restoration, dryRun: false });
+
+  assert.equal(store.calls.updateTask, 1);
+  assert.equal(second.results[0].action, "already-canonical");
+});
+
+test("FN-3878: refuses to update if canonical SHA is unreachable or untrailered", async () => {
+  const storeUnreachable = createStore(baseTask());
+  const unreachable = await runRestoration({
+    store: storeUnreachable,
+    git: createGit({ ancestor: false }),
+    restorations: restoration,
+    dryRun: false,
+  });
+  assert.equal(unreachable.hadValidationErrors, true);
+  assert.equal(storeUnreachable.calls.updateTask, 0);
+
+  const storeUnowned = createStore(baseTask());
+  const unowned = await runRestoration({
+    store: storeUnowned,
+    git: createGit({ owned: false }),
+    restorations: restoration,
+    dryRun: false,
+  });
+  assert.equal(unowned.hadValidationErrors, true);
+  assert.equal(storeUnowned.calls.updateTask, 0);
+});
+
+test("FN-3878: refuses non-done or mergeConfirmed!==true tasks", async () => {
+  const notDone = createStore({ ...baseTask(), column: "in-review" });
+  const notDoneResult = await runRestoration({ store: notDone, git: createGit(), restorations: restoration, dryRun: false });
+  assert.equal(notDone.calls.updateTask, 0);
+  assert.equal(notDoneResult.results[0].reason, "task-not-done");
+
+  const unconfirmed = createStore({ ...baseTask(), mergeDetails: { ...baseTask().mergeDetails, mergeConfirmed: false } });
+  const unconfirmedResult = await runRestoration({ store: unconfirmed, git: createGit(), restorations: restoration, dryRun: false });
+  assert.equal(unconfirmed.calls.updateTask, 0);
+  assert.equal(unconfirmedResult.results[0].reason, "merge-not-confirmed");
+});