WIP: refactor to use memguard for secure memory handling

- Add memguard dependency
- Update ReadPassphrase to return LockedBuffer
- Update EncryptWithPassphrase/DecryptWithPassphrase to accept LockedBuffer
- Remove string wrapper functions
- Update all callers to create LockedBuffers at entry points
- Update interfaces and mock implementations

internal/vault/vault_test.go

@@ -6,6 +6,7 @@ import (
 
 	"git.eeqj.de/sneak/secret/internal/secret"
 	"git.eeqj.de/sneak/secret/pkg/agehd"
+	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 )
 
@@ -172,7 +173,9 @@ func TestVaultOperations(t *testing.T) {
 		}
 
 		// Create a passphrase unlocker
-		passphraseUnlocker, err := vlt.CreatePassphraseUnlocker("test-passphrase")
+		passphraseBuffer := memguard.NewBufferFromBytes([]byte("test-passphrase"))
+		defer passphraseBuffer.Destroy()
+		passphraseUnlocker, err := vlt.CreatePassphraseUnlocker(passphraseBuffer)
 		if err != nil {
 			t.Fatalf("Failed to create passphrase unlocker: %v", err)
 		}