test: eliminate chokidar/FSEvents race in watcher + staleness-banner tests (#434)

Mocks chokidar at the module level for `__tests__/watcher.test.ts` and
`__tests__/mcp-staleness-banner.test.ts` so the pending-file-tracking and
staleness-banner tests no longer depend on OS-level file-watcher delivery
latency. Reduces full-suite failure rate from 3/10 to 0/10.

- `__tests__/__helpers__/chokidar-mock.ts` (new) — controllable
  EventEmitter; `chokidarMockModule` for `vi.mock('chokidar', ...)` plus
  `triggerFileEvent(root, event, relPath)` helper. `watch()` returns an
  EventEmitter that fires `ready` on the next microtask.
- `__tests__/watcher.test.ts` — refactors every event-driving test to
  use `triggerFileEvent` instead of `fs.writeFileSync` for the trigger.
  Pending-file tests assert state synchronously. Filtering tests still
  verify FileWatcher's own filter chain.
- `__tests__/mcp-staleness-banner.test.ts` — same vi.mock + same
  `triggerFileEvent` pattern; tests keep `fs.writeFileSync` for on-disk
  content (`cg.sync()` needs the bytes) and add the synthesized event
  on top.

The watcher's debounce timer (real `setTimeout`) is left untouched — that's
the unit under test.

Total test count unchanged (928 passing + 2 pre-existing skips). Wall-clock
runtime improved (no more 8000ms waitFor polls against real chokidar).

One disclosed tradeoff: the previous node_modules filtering test
incidentally exercised chokidar's `ignored` callback at the OS level;
with chokidar mocked, that property of chokidar itself isn't covered
here. Commented inline.