Integrate afero filesystem abstraction library

- Add afero.Fs field to Vaultik struct for filesystem operations
- Vaultik now owns and manages the filesystem instance
- SnapshotManager receives filesystem via SetFilesystem() setter
- Update blob packer to use afero for temporary files
- Convert all filesystem operations to use afero abstraction
- Remove filesystem module - Vaultik manages filesystem directly
- Update tests: remove symlink test (unsupported by afero memfs)
- Fix TestMultipleFileChanges to handle scanner examining directories

This enables full end-to-end testing without touching disk by using
memory-backed filesystems. Database operations continue using real
filesystem as SQLite requires actual files.

internal/snapshot/scanner.go

@@ -93,6 +93,7 @@ func NewScanner(cfg ScannerConfig) *Scanner {
 		CompressionLevel: cfg.CompressionLevel,
 		Recipients:       cfg.AgeRecipients,
 		Repositories:     cfg.Repositories,
+		Fs:               cfg.FS,
 	}
 	packer, err := blob.NewPacker(packerCfg)
 	if err != nil {