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.mod

@@ -13,6 +13,7 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.90.0
 	github.com/aws/smithy-go v1.23.2
 	github.com/dustin/go-humanize v1.0.1
+	github.com/fsnotify/fsnotify v1.9.0
 	github.com/gobwas/glob v0.2.3
 	github.com/google/uuid v1.6.0
 	github.com/johannesboyne/gofakes3 v0.0.0-20250603205740-ed9094be7668