File-system watcher for config reload triggers.

Uses the file_system library (when available) to watch config files for changes. Falls back to periodic polling when the native watcher is not available or fails to start.

Watched paths:

• ~/.lemon/config.toml
• <cwd>/.lemon/config.toml

Both come from LemonCore.Paths, so pointing the config elsewhere moves the watched paths with it.

• .env (from LEMON_DOTENV_DIR or cwd)

Native watcher targets are optimized:

• watch exact file paths when they already exist
• watch parent directories when files do not exist yet

File-system events are still filtered to the exact watched file set and debounced (250ms) to avoid redundant reloads from editor save sequences.