fix: file @ autocomplete performance using fd

- Replace slow synchronous directory walking with fd for fuzzy file search
- Auto-download fd to ~/.pi/agent/tools/ if not found in PATH
- Performance improved from ~900ms to ~10ms per keystroke on large repos
- Remove minimatch dependency from tui package
- Graceful degradation if fd unavailable (empty results)

Fixes #69

.gitignore

@@ -3,7 +3,7 @@ dist/
 *.log
 .DS_Store
 *.tsbuildinfo
-packages/*/node_modules/
+# packages/*/node_modules/
 packages/*/dist/
 packages/*/dist-chrome/
 packages/*/dist-firefox/