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

go.sum

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