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>