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/blob/packer_test.go

@@ -13,6 +13,7 @@ import (
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/log"
 	"github.com/klauspost/compress/zstd"
+	"github.com/spf13/afero"
 )
 
 const (
@@ -45,6 +46,7 @@ func TestPacker(t *testing.T) {
 			CompressionLevel: 3,
 			Recipients:       []string{testPublicKey},
 			Repositories:     repos,
+			Fs:               afero.NewMemMapFs(),
 		}
 		packer, err := NewPacker(cfg)
 		if err != nil {
@@ -134,6 +136,7 @@ func TestPacker(t *testing.T) {
 			CompressionLevel: 3,
 			Recipients:       []string{testPublicKey},
 			Repositories:     repos,
+			Fs:               afero.NewMemMapFs(),
 		}
 		packer, err := NewPacker(cfg)
 		if err != nil {
@@ -216,6 +219,7 @@ func TestPacker(t *testing.T) {
 			CompressionLevel: 3,
 			Recipients:       []string{testPublicKey},
 			Repositories:     repos,
+			Fs:               afero.NewMemMapFs(),
 		}
 		packer, err := NewPacker(cfg)
 		if err != nil {
@@ -304,6 +308,7 @@ func TestPacker(t *testing.T) {
 			CompressionLevel: 3,
 			Recipients:       []string{testPublicKey},
 			Repositories:     repos,
+			Fs:               afero.NewMemMapFs(),
 		}
 		packer, err := NewPacker(cfg)
 		if err != nil {