feat: Add file watcher with debounced auto-sync and comprehensive test coverage

Addresses the need for automatic graph synchronization on file changes. Implements FileWatcher using native OS file events (FSEvents/inotify/ReadDirectoryChangesW) with 2-second debouncing to prevent thrashing on rapid saves. Filters changes against include/exclude patterns and ignores .codegraph directory modifications. Integrates with CodeGraph API (watch/unwatch/isWatching methods) and MCP server for automatic activation. Updates documentation to reflect shift from semantic to full-text search and removal of manual hook installation requirements.

src/sync/index.ts

@@ -4,14 +4,10 @@
  * Provides synchronization functionality for keeping the code graph
  * up-to-date with file system changes.
  *
- * Note: Git hooks functionality has been removed. CodeGraph sync is now
- * triggered through codegraph's Claude Code hooks integration instead.
- *
  * Components:
+ * - FileWatcher: Debounced fs.watch that auto-triggers sync on file changes
  * - Content hashing for change detection (in extraction module)
  * - Incremental reindexing (in extraction module)
  */
 
-// This module is kept for potential future sync-related exports
-// Currently all sync functionality is in the extraction module
-export {};
+export { FileWatcher, WatchOptions } from './watcher';