feat(FN-4948): complete Step 2 — wire identity guard into worktree provisioning

Fusion-Task-Id: FN-4948
Fusion-Task-Lineage: dc622643-4c3e-4217-9219-a6a6e5424427

packages/engine/src/__tests__/worktree-acquisition-backend.test.ts

@@ -1,4 +1,8 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("../worktree-hooks.js", () => ({
+  installTaskWorktreeIdentityGuard: vi.fn().mockResolvedValue(undefined),
+}));
 import { acquireTaskWorktree } from "../worktree-acquisition.js";
 import type { WorktreeBackend } from "../worktree-backend.js";