feat: implement daemon mode with filesystem watching

Replace the daemon mode stub with a full implementation that:

- Watches configured snapshot paths for filesystem changes using
  fsnotify (inotify on Linux, FSEvents on macOS, etc.)
- Runs an initial full backup on startup
- Triggers incremental backups at backup_interval when changes are
  detected, only for snapshots whose paths were affected
- Performs full periodic scans at full_scan_interval regardless of
  detected changes
- Respects min_time_between_run to prevent excessive backup runs
- Handles SIGTERM/SIGINT for graceful shutdown, completing any
  in-progress backup before exiting
- Automatically watches newly created subdirectories
- Uses a backup semaphore to prevent concurrent backup runs

New files:
- internal/vaultik/daemon.go: RunDaemon(), changeTracker, watcher setup
- internal/vaultik/daemon_test.go: Tests for changeTracker, isSubpath,
  concurrency safety, and daemon constants

closes #3

TODO.md

@@ -106,23 +106,21 @@ User must have rclone configured separately (via `rclone config`).
 
 ---
 
-## Post-1.0 (Daemon Mode)
+## Daemon Mode (Complete)
 
-1. Implement inotify file watcher for Linux
-   - Watch source directories for changes
-   - Track dirty paths in memory
+1. [x] Implement cross-platform filesystem watcher (via fsnotify)
+   - Watches source directories for changes
+   - Tracks dirty paths in memory
+   - Automatically watches new directories
 
-1. Implement FSEvents watcher for macOS
-   - Watch source directories for changes
-   - Track dirty paths in memory
+1. [x] Implement backup scheduler in daemon mode
+   - Respects backup_interval config
+   - Triggers backup when dirty paths exist and interval elapsed
+   - Implements full_scan_interval for periodic full scans
+   - Respects min_time_between_run to prevent excessive runs
 
-1. Implement backup scheduler in daemon mode
-   - Respect backup_interval config
-   - Trigger backup when dirty paths exist and interval elapsed
-   - Implement full_scan_interval for periodic full scans
-
-1. Add proper signal handling for daemon
+1. [x] Add proper signal handling for daemon
    - Graceful shutdown on SIGTERM/SIGINT
-   - Complete in-progress backup before exit
+   - Completes in-progress backup before exit
 
-1. Write tests for daemon mode
+1. [x] Write tests for daemon mode