fix(git-sync): git-http stream error handlers + close test gaps (#119 review)

Addresses the stability + test-coverage warnings from the #119 review:

- git-http-backend.service.ts: add `'error'` handlers to child.stdout/stderr. An
  EventEmitter 'error' with no listener (e.g. EPIPE when the client aborts
  mid-response) is rethrown by Node as an uncaught exception and crashes the
  process; now swallowed + logged (never echoed to the client).
- TEST INFRA: a jest setupFile shims `navigator`/`MessageChannel` for the `node`
  testEnvironment. react-dom@18 reads `navigator` at module-init (pulled in via
  @docmost/editor-ext -> @tiptap/react), so every spec transitively importing the
  conversion engine — including git-http.service.spec.ts — previously FAILED TO
  LOAD ("navigator is not defined") and ran ZERO tests. With the shim those specs
  now run (git-sync integration: 11 suites / 133 tests green).
- git-http.service.spec.ts: cover the 503 lock-held push path — `ingestExternalPush`
  rejecting `GitSyncLockHeldError` -> 503 + Retry-After + "git-sync busy, retry",
  no double header write (+ the already-headers-sent no-rewrite path).
- git-http-backend.service.spec.ts: unit-test run() — child 'error'/'close' before
  headers -> 500; normal CGI parse+stream; stdout/stderr 'error' (EPIPE) swallowed;
  synchronous spawn throw -> 500.
- page-change.listener.ts: implement OnModuleDestroy to clearTimeout all pending
  debounce timers on shutdown (+ test).
- .env.example: vaults are non-bare working repos, not "bare repos".

(Docs deleted by the stray commit were restored in 9cdbce54.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -210,7 +210,8 @@ MCP_DOCMOST_PASSWORD=
 # operations (create / move / rename / delete) are attributed to.
 # GIT_SYNC_SERVICE_USER_ID=
 #
-# Where the per-space bare repos / working vaults live.
+# Where the per-space working vaults live (non-bare repos; the engine needs a
+# working tree).
 # Defaults to "<DATA_DIR or ./data>/git-sync".
 # GIT_SYNC_DATA_DIR=
 #