fix(mcp): skip fs.watch on WSL2 /mnt drives that hang MCP startup (#199) (#210)

Recursive fs.watch on a WSL2 /mnt NTFS/9p mount walks the directory tree
with every readdir/stat crossing the Windows boundary, stalling the event
loop long enough to blow past opencode's 30s MCP handshake timeout so the
tools never appear. This is the file-watcher half of the #172 fix, which
moved the DB/WASM open off the handshake but left the watcher on the
critical path.

- Add watchDisabledReason() policy: CODEGRAPH_NO_WATCH (off) >
  CODEGRAPH_FORCE_WATCH (force on) > WSL2 + /mnt auto-detect (off).
  FileWatcher.start() and the MCP server both honor it; the server now
  logs why watching is off and how to refresh.
- Add `codegraph serve --mcp --no-watch`.
- When watching is off, init/install offer git sync hooks (post-commit,
  post-merge, post-checkout) that run `codegraph sync` in the background,
  or fall back to manual sync; either way the user is told the index
  stays frozen until re-synced. uninit removes the hooks.
- Tests: watch-policy + git-hooks (idempotency, user-content preservation,
  core.hooksPath).

Root-cause analysis and workaround by @mengfanbo123.

Closes #199

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/sync/index.ts

@@ -6,8 +6,20 @@
  *
  * Components:
  * - FileWatcher: Debounced fs.watch that auto-triggers sync on file changes
+ * - Watch policy: decides when the watcher must be disabled (e.g. WSL2 /mnt)
+ * - Git sync hooks: opt-in commit/merge/checkout hooks when watching is off
  * - Content hashing for change detection (in extraction module)
  * - Incremental reindexing (in extraction module)
  */
 
 export { FileWatcher, WatchOptions } from './watcher';
+export { watchDisabledReason, detectWsl } from './watch-policy';
+export {
+  installGitSyncHook,
+  removeGitSyncHook,
+  isSyncHookInstalled,
+  isGitRepo,
+  DEFAULT_SYNC_HOOKS,
+  type GitHookName,
+  type GitHookResult,
+} from './git-hooks';