fix(FN-5256): route merger-handoff reconcile through hardened gates

Code-review follow-up to cf0101be7. The merger integration worktree path
was still calling bare `activeSessionRegistry.reconcileStaleSelfOwned` for
same-task entries, bypassing the minIdleMs window introduced by the main
fix. A merger-handoff that races a warming-down session could clear a
registry entry < 5s old. Route through `reconcileSelfOwnedActiveSessionForRemoval`
with the `executingTaskLock` process probe so all reconcile sites enforce
the same liveness contract. Test updated to backdate `registeredAt`,
matching the pattern used for the other reliability-interactions tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/merger-integration-worktree.test.ts

@@ -304,6 +304,8 @@ describe("acquireReuseHandoff", () => {
       kind: "executor",
       ownerKey: "FN-5279",
     });
+    // FN-5256: backdate so the new min-idle window doesn't refuse the reconcile.
+    (activeSessionRegistry.lookupByPath("/tmp/task-worktree") as any).registeredAt = 0;
 
     await acquireReuseHandoff({
       task: await store.getTask("FN-5279"),

packages/engine/src/merger-integration-worktree.ts

@@ -8,7 +8,11 @@ import type {
   Task,
   TaskStore,
 } from "@fusion/core";
-import { activeSessionRegistry, executingTaskLock } from "./active-session-registry.js";
+import {
+  activeSessionRegistry,
+  executingTaskLock,
+  reconcileSelfOwnedActiveSessionForRemoval,
+} from "./active-session-registry.js";
 import { attemptBranchAutocorrect } from "./branch-autocorrect.js";
 import { MeshLeaseManager } from "./mesh-lease-manager.js";
 import {
@@ -294,14 +298,24 @@ export async function acquireReuseHandoff(input: ReuseHandoffInput): Promise<Han
 
   const activeRecord = activeSessionRegistry.lookupByPath(worktreePath);
   if (activeRecord) {
-    if (activeRecord.taskId === input.task.id && !executingTaskLock.has(input.task.id)) {
-      const reconciled = activeSessionRegistry.reconcileStaleSelfOwned(worktreePath, input.task.id);
-      if (!reconciled.reconciled) {
+    if (activeRecord.taskId === input.task.id) {
+      // FN-5256: route through the hardened helper so the minIdleMs window and
+      // processActiveProbe (executingTaskLock) gates apply — bare
+      // `reconcileStaleSelfOwned` would race a warming-down session.
+      const outcome = reconcileSelfOwnedActiveSessionForRemoval(
+        activeSessionRegistry,
+        worktreePath,
+        input.task.id,
+        () => false,
+        { processActiveProbe: (probeTaskId) => executingTaskLock.has(probeTaskId) },
+      );
+      if (outcome.action !== "reconciled") {
         throw new MergeHandoffRefusedError("active-session-binding", "active-session-present", {
           taskId: input.task.id,
           worktreePath,
           activeRecord,
           executingTaskLockHeld: executingTaskLock.has(input.task.id),
+          reconcileOutcome: outcome.action,
         });
       }
     } else {