feat(FN-5348): remove cwd-main integration mode fallback

Removes the `cwd-main` integration fallback mode (FN-5348), eliminating the legacy shortcut path where the merger would operate directly on the project root instead of a dedicated worktree. Steps normalize the `reuse-task-worktree` integration mode as the sole path, wire stricter mode invariants in

Fusion-Task-Id: FN-5348

packages/engine/src/__tests__/reliability-interactions/cwd-integration-fallback-removed.test.ts

@@ -0,0 +1,57 @@
+import { mkdir } from "node:fs/promises";
+import { join } from "node:path";
+import { describe, expect, it } from "vitest";
+
+import { aiMergeTask } from "../../merger.js";
+import { git, hasGit, makeReliabilityFixture } from "./_helpers.js";
+
+describe("FN-5348 reliability interactions: cwd fallback removal", () => {
+  it.skipIf(!hasGit)("autoMerge=false + reuse refusal stays in-review and emits no cwd fallback events", async () => {
+    const fixture = await makeReliabilityFixture({
+      taskId: "FN-5348-RI-AUTO-OFF-REFUSAL",
+      settings: {
+        autoMerge: false,
+        baseBranch: "master",
+        mergeIntegrationWorktree: "reuse-task-worktree",
+      } as any,
+    });
+
+    try {
+      const { rootDir, store, task } = fixture;
+      const actualTask = await store.getTask(task.id);
+      const branch = `fusion/${actualTask!.id.toLowerCase()}`;
+      const worktreeRoot = `${rootDir}-worktrees`;
+      const worktreePath = join(worktreeRoot, actualTask!.id.toLowerCase());
+
+      git(rootDir, "git branch -m main master");
+      const completedSteps = (actualTask?.steps ?? []).map((step) => ({ ...step, status: "done" as const }));
+      await store.updateTask(task.id, {
+        baseBranch: "master",
+        branch,
+        steps: completedSteps,
+        currentStep: completedSteps.length,
+      } as any);
+      await fixture.createBranch(branch);
+      await fixture.writeAndCommit("packages/engine/src/fn-5348-ri-refusal.ts", "export const refusal = true;\n", "feat: add refusal merge content");
+      await fixture.checkout("master");
+      await mkdir(worktreeRoot, { recursive: true });
+      git(rootDir, `git worktree add ${JSON.stringify(worktreePath)} ${JSON.stringify(branch)}`);
+      await store.updateTask(task.id, { worktree: worktreePath, branch } as any);
+      store.enqueueMergeQueue(task.id);
+      git(worktreePath, "sh -c 'printf dirty > DIRTY.txt'");
+
+      await expect(aiMergeTask(store, rootDir, task.id)).rejects.toMatchObject({
+        name: "MergeHandoffRefusedError",
+        gate: "working-tree-dirty",
+      });
+
+      const latest = await store.getTask(task.id);
+      expect(latest?.column).toBe("in-review");
+      const auditTypes = store.getRunAuditEvents({ taskId: task.id }).map((event) => event.mutationType);
+      expect(auditTypes).toContain("merge:reuse-handoff-refused");
+      expect(auditTypes).not.toContain("merge:cwd-integration-fallback-removed");
+    } finally {
+      await fixture.cleanup();
+    }
+  }, 30_000);
+});

packages/engine/src/__tests__/reliability-interactions/merge-reuse-task-worktree.test.ts

@@ -438,7 +438,7 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
     }
   }, 30_000);
 
-  it.skipIf(!hasGit)("worktrunk override records deferred-to-worktrunk without acquiring reuse handoff", async () => {
+  it.skipIf(!hasGit)("worktrunk-enabled reuse mode still acquires reuse handoff", async () => {
     const fixture = await makeReliabilityFixture({
       taskId: "FN-5279-RI-WORKTRUNK",
       settings: {
@@ -452,6 +452,8 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
       const { rootDir, store, task } = fixture;
       const actualTask = await store.getTask(task.id);
       const branch = `fusion/${actualTask!.id.toLowerCase()}`;
+      const worktreeRoot = `${rootDir}-worktrees`;
+      const worktreePath = join(worktreeRoot, actualTask!.id.toLowerCase());
 
       git(rootDir, "git branch -m main master");
       const completedSteps = (actualTask?.steps ?? []).map((step) => ({ ...step, status: "done" as const }));
@@ -464,12 +466,16 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
       await fixture.createBranch(branch);
       await fixture.writeAndCommit("packages/engine/src/fn-5279-ri-worktrunk.ts", "export const deferred = true;\n", "feat: add worktrunk merge content");
       await fixture.checkout("master");
+      await mkdir(worktreeRoot, { recursive: true });
+      git(rootDir, `git worktree add ${JSON.stringify(worktreePath)} ${JSON.stringify(branch)}`);
+      await store.updateTask(task.id, { worktree: worktreePath, branch } as any);
+      store.enqueueMergeQueue(task.id);
 
       const result = await aiMergeTask(store, rootDir, task.id);
       expect(result.merged).toBe(true);
       const auditTypes = store.getRunAuditEvents({ taskId: task.id }).map((event) => event.mutationType);
       expect(auditTypes).toContain("merge:reuse-handoff-deferred-to-worktrunk");
-      expect(auditTypes).not.toContain("merge:reuse-handoff-acquired");
+      expect(auditTypes).toContain("merge:reuse-handoff-acquired");
     } finally {
       await fixture.cleanup();
     }