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-worktrunk.test.ts

@@ -10,6 +10,7 @@ vi.mock("node:child_process", () => ({ exec: execMock }));
 vi.mock("node:fs", () => ({ existsSync: existsSyncMock }));
 vi.mock("../worktree-hooks.js", () => ({
   installTaskWorktreeIdentityGuard: vi.fn().mockResolvedValue(undefined),
+  IDENTITY_GUARD_BYPASS_ENV: "FUSION_MERGER_BYPASS_IDENTITY_GUARD",
 }));
 vi.mock("../worktree-pool.js", async () => {
   const actual = await vi.importActual<any>("../worktree-pool.js");