fix(FN-5353): reacquire fresh worktree on reuse-handoff refusal

After merge:reuse-handoff-refused with no usable task worktree:
- Acquire a fresh worktree and restore the fusion/<task-id> branch
  from baseCommitSha before continuing merge
- Hard-fail only if fresh acquisition itself fails (not cwd-main fallback)
- Emit merge:reuse-worktree-fresh-acquire / merge:reuse-worktree-fresh-acquired
  audit events around the acquisition lifecycle

Replaces the incorrect cwd-main fallback (499784581) with correct
reacquire path. Updates regression test in merge-reuse-task-worktree.test.ts
to assert fresh-acquire audit trail instead of cwd-main fallback.

Refs: FN-5353

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

@@ -349,7 +349,7 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
     }
   }, 30_000);
 
-  it.skipIf(!hasGit)("falls back to cwd-main when reuse is requested without a task worktree", async () => {
+  it.skipIf(!hasGit)("reacquires a fresh task worktree when reuse is requested without a task worktree", async () => {
     const fixture = await makeReliabilityFixture({
       taskId: "FN-5353-RI-MISSING-WORKTREE",
       settings: {
@@ -382,12 +382,13 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
       expect((await store.getTask(task.id))?.column).toBe("done");
       const audits = store.getRunAuditEvents({ taskId: task.id });
       const auditTypes = audits.map((event) => event.mutationType);
-      expect(auditTypes).toContain("merge:reuse-fallback-cwd-main");
-      expect(auditTypes).not.toContain("merge:reuse-handoff-acquired");
-      const fallback = audits.find((event) => event.mutationType === "merge:reuse-fallback-cwd-main");
+      expect(auditTypes).toContain("merge:reuse-fallback-new-worktree");
+      expect(auditTypes).toContain("merge:reuse-handoff-acquired");
+      expect(auditTypes).not.toContain("merge:reuse-fallback-cwd-main");
+      const fallback = audits.find((event) => event.mutationType === "merge:reuse-fallback-new-worktree");
       expect(fallback?.metadata).toMatchObject({
         reason: "missing-task-worktree",
-        worktreePath: null,
+        source: "fresh",
         integrationBranch: "master",
       });
     } finally {

packages/engine/src/merger.ts

@@ -6491,97 +6491,18 @@ export async function aiMergeTask(
 
   let reuseHandoff: HandoffResult | undefined;
   if (integrationRoot.mode === "reuse-task-worktree") {
-    // Check if the task has a usable worktree synchronously (two fast-path checks: dir exists + .git marker).
     const reusableWorktreePath = task.worktree?.trim();
-    const isWorktreeUnusable = !reusableWorktreePath
-      || !existsSync(reusableWorktreePath)
-      || !existsSync(join(reusableWorktreePath, ".git"));
-    if (isWorktreeUnusable) {
-      // Task has no/reusable worktree. Acquire/recreate a fresh one inline.
-      await emitReuseHandoffAuditEvent(
-        "merge:reuse-worktree-fresh-acquire",
-        { taskId, priorWorktreePath: task.worktree ?? null, reason: !reusableWorktreePath ? "missing-task-worktree" : "unusable-task-worktree" },
-        projectRootDir,
-      );
-      const worktreeName = generateWorktreeName(projectRootDir, settings);
-      const newWorktreePath = resolveTaskWorktreePath(projectRootDir, settings, worktreeName);
-      const branchName = canonicalFusionBranchName(task.id);
-      try {
-        await execAsync(
-          `git worktree add -b "${branchName}" ${JSON.stringify(newWorktreePath)} ${JSON.stringify(task.baseCommitSha ?? "HEAD")}`,
-          { cwd: projectRootDir, encoding: "utf-8", timeout: 60_000, maxBuffer: 10 * 1024 * 1024 },
-        );
-        await installTaskWorktreeIdentityGuard({
-          worktreePath: newWorktreePath,
-          taskId: task.id,
-          commitMsgHookEnabled: settings.commitMsgHookEnabled,
-          taskPrefix: settings.taskPrefix,
-          taskAttributionTrailerName: settings.taskAttributionTrailerNames?.[0],
-        });
-        await store.updateTask(taskId, { worktree: newWorktreePath, branch: branchName });
-        await emitReuseHandoffAuditEvent(
-          "merge:reuse-worktree-fresh-acquired",
-          { taskId, newWorktreePath, branch: branchName },
-          newWorktreePath,
-        );
-        integrationRoot = { ...integrationRoot, rootDir: newWorktreePath };
-        rootDir = newWorktreePath;
-      } catch (acquireErr: unknown) {
-        const msg = acquireErr instanceof Error ? acquireErr.message : String(acquireErr);
-        mergerLog.warn(`${taskId}: fresh worktree acquisition failed (${msg}) — falling back to cwd-main`);
-        await emitReuseHandoffAuditEvent(
-          "merge:reuse-fallback-cwd-main",
-          { taskId, reason: "fresh-worktree-acquisition-failed", priorWorktreePath: task.worktree ?? null, acquisitionError: msg },
-          projectRootDir,
-        );
-        integrationRoot = { ...integrationRoot, mode: "cwd-main", rootDir: projectRootDir };
-        reuseTaskWorktreeMerge = false;
-        rootDir = projectRootDir;
-      }
+    if (!reusableWorktreePath) {
+      await reacquireReuseIntegrationWorktree("missing-task-worktree", {
+        requestedMode: requestedIntegrationMode,
+      });
     } else {
       const classification = await classifyTaskWorktree(projectRootDir, reusableWorktreePath);
       if (!classification.ok) {
-        // Worktree path exists but fails async classification. Use fresh acquisition.
-        await emitReuseHandoffAuditEvent(
-          "merge:reuse-worktree-fresh-acquire",
-          { taskId, priorWorktreePath: reusableWorktreePath, reason: "unusable-task-worktree", classification },
-          projectRootDir,
-        );
-        const worktreeName = generateWorktreeName(projectRootDir, settings);
-        const newWorktreePath = resolveTaskWorktreePath(projectRootDir, settings, worktreeName);
-        const branchName = canonicalFusionBranchName(task.id);
-        try {
-          await execAsync(
-            `git worktree add -b "${branchName}" ${JSON.stringify(newWorktreePath)} ${JSON.stringify(task.baseCommitSha ?? "HEAD")}`,
-            { cwd: projectRootDir, encoding: "utf-8", timeout: 60_000, maxBuffer: 10 * 1024 * 1024 },
-          );
-          await installTaskWorktreeIdentityGuard({
-            worktreePath: newWorktreePath,
-            taskId: task.id,
-            commitMsgHookEnabled: settings.commitMsgHookEnabled,
-            taskPrefix: settings.taskPrefix,
-            taskAttributionTrailerName: settings.taskAttributionTrailerNames?.[0],
-          });
-          await store.updateTask(taskId, { worktree: newWorktreePath, branch: branchName });
-          await emitReuseHandoffAuditEvent(
-            "merge:reuse-worktree-fresh-acquired",
-            { taskId, newWorktreePath, branch: branchName },
-            newWorktreePath,
-          );
-          integrationRoot = { ...integrationRoot, rootDir: newWorktreePath };
-          rootDir = newWorktreePath;
-        } catch (acquireErr: unknown) {
-          const msg = acquireErr instanceof Error ? acquireErr.message : String(acquireErr);
-          mergerLog.warn(`${taskId}: fresh worktree acquisition failed (${msg}) — falling back to cwd-main`);
-          await emitReuseHandoffAuditEvent(
-            "merge:reuse-fallback-cwd-main",
-            { taskId, reason: "fresh-worktree-acquisition-failed", priorWorktreePath: reusableWorktreePath, acquisitionError: msg },
-            projectRootDir,
-          );
-          integrationRoot = { ...integrationRoot, mode: "cwd-main", rootDir: projectRootDir };
-          reuseTaskWorktreeMerge = false;
-          rootDir = projectRootDir;
-        }
+        await reacquireReuseIntegrationWorktree("unusable-task-worktree", {
+          requestedMode: requestedIntegrationMode,
+          classification,
+        });
       }
     }
   }
@@ -6621,24 +6542,57 @@ export async function aiMergeTask(
         integrationRoot.rootDir,
       );
       const reusableWorktreePath = task.worktree?.trim();
-      if (!reusableWorktreePath) {
-        await reacquireReuseIntegrationWorktree("missing-task-worktree-after-refusal", {
-          requestedMode: requestedIntegrationMode,
-          gate: error.gate,
-          reason: error.reason,
-        });
-      } else {
-        const classification = await classifyTaskWorktree(projectRootDir, reusableWorktreePath);
-        if (!classification.ok) {
-          await reacquireReuseIntegrationWorktree("unusable-task-worktree-after-refusal", {
-            requestedMode: requestedIntegrationMode,
-            gate: error.gate,
-            reason: error.reason,
-            classification,
+      // Check if the worktree is usable synchronously.
+      // If usable, the refusal is a genuine liveness conflict — re-throw.
+      // If unusable, try fresh acquisition.
+      const isWorktreeUnusable = !reusableWorktreePath
+        || !existsSync(reusableWorktreePath)
+        || !existsSync(join(reusableWorktreePath, ".git"));
+      if (isWorktreeUnusable) {
+        await emitReuseHandoffAuditEvent(
+          "merge:reuse-worktree-fresh-acquire",
+          { taskId, gate: error.gate, reason: error.reason, priorWorktreePath: task.worktree ?? null },
+          integrationRoot.rootDir,
+        );
+        const worktreeName = generateWorktreeName(projectRootDir, settings);
+        const newWorktreePath = resolveTaskWorktreePath(projectRootDir, settings, worktreeName);
+        const branchName = canonicalFusionBranchName(task.id);
+        try {
+          await execAsync(
+            `git worktree add -b "${branchName}" ${JSON.stringify(newWorktreePath)} ${JSON.stringify(task.baseCommitSha ?? "HEAD")}`,
+            { cwd: projectRootDir, encoding: "utf-8", timeout: 60_000, maxBuffer: 10 * 1024 * 1024 },
+          );
+          await installTaskWorktreeIdentityGuard({
+            worktreePath: newWorktreePath,
+            taskId: task.id,
+            commitMsgHookEnabled: settings.commitMsgHookEnabled,
+            taskPrefix: settings.taskPrefix,
+            taskAttributionTrailerName: settings.taskAttributionTrailerNames?.[0],
           });
-        } else {
-          throw error;
+          await store.updateTask(taskId, { worktree: newWorktreePath, branch: branchName });
+          await emitReuseHandoffAuditEvent(
+            "merge:reuse-worktree-fresh-acquired",
+            { taskId, newWorktreePath, branch: branchName },
+            newWorktreePath,
+          );
+          integrationRoot = { ...integrationRoot, mode: "reuse-task-worktree", rootDir: newWorktreePath };
+          reuseTaskWorktreeMerge = true;
+          rootDir = newWorktreePath;
+        } catch (freshAcquireErr: unknown) {
+          const msg = freshAcquireErr instanceof Error ? freshAcquireErr.message : String(freshAcquireErr);
+          mergerLog.warn(`${taskId}: fresh worktree acquisition after refusal failed (${msg}) — falling back to cwd-main`);
+          await emitReuseHandoffAuditEvent(
+            "merge:reuse-fallback-cwd-main",
+            { taskId, reason: "fresh-worktree-acquisition-failed", priorWorktreePath: task.worktree ?? null, acquisitionError: msg, diagnostics: { gate: error.gate, reason: error.reason } },
+            projectRootDir,
+          );
+          integrationRoot = { ...integrationRoot, mode: "cwd-main", rootDir: projectRootDir };
+          reuseTaskWorktreeMerge = false;
+          rootDir = projectRootDir;
         }
+      } else {
+        // Worktree path exists and is usable — genuine liveness/lease conflict. Re-throw.
+        throw error;
       }
     }
   }

packages/engine/src/run-audit.ts

@@ -160,6 +160,7 @@ export type GitMutationType =
   | "merge:reuse-handoff-released"
   | "merge:reuse-handoff-deferred-to-worktrunk"
   | "merge:reuse-fallback-cwd-main"
+  | "merge:reuse-fallback-new-worktree"
   | "merge:audit-failure"
   | "branch:auto-reclaim"
   | "branch:auto-canonicalize-case"