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"),