feat(FN-5479): fix identity-guard merger bypass on detached HEAD and gate l

The merge delivers the FN-5483 identity-guard bypass for merger-driven commits on detached HEAD, plus Steps 2 and 6 of FN-5479 which gate the limbo counter by enqueue acceptance and document the associated invariant. It also restores the dashboard's PWA and theme-boot contract in index.html. New reg

Fusion-Task-Id: FN-5479

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

@@ -2,6 +2,7 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 
 vi.mock("../worktree-hooks.js", () => ({
   installTaskWorktreeIdentityGuard: vi.fn().mockResolvedValue(undefined),
+  IDENTITY_GUARD_BYPASS_ENV: "FUSION_MERGER_BYPASS_IDENTITY_GUARD",
 }));
 import { acquireTaskWorktree } from "../worktree-acquisition.js";
 import type { WorktreeBackend } from "../worktree-backend.js";