Fix DecryptWithIdentity to return LockedBuffer

- Changed DecryptWithIdentity to return *memguard.LockedBuffer instead of []byte
- Updated all callers throughout the codebase to handle LockedBuffer
- This ensures decrypted data is protected in memory immediately after decryption
- Fixed all usages in vault, secret, version, and unlocker implementations
- Removed duplicate buffer creation and unnecessary memory clearing

internal/vault/vault.go

@@ -157,22 +157,23 @@ func (v *Vault) GetOrDeriveLongTermKey() (*age.X25519Identity, error) {
 
 	// Decrypt long-term private key using unlocker
 	secret.Debug("Decrypting long-term private key with unlocker", "unlocker_type", unlocker.GetType())
-	ltPrivKeyData, err := secret.DecryptWithIdentity(encryptedLtPrivKey, unlockerIdentity)
+	ltPrivKeyBuffer, err := secret.DecryptWithIdentity(encryptedLtPrivKey, unlockerIdentity)
 	if err != nil {
 		secret.Debug("Failed to decrypt long-term private key", "error", err, "unlocker_type", unlocker.GetType())
 
 		return nil, fmt.Errorf("failed to decrypt long-term private key: %w", err)
 	}
+	defer ltPrivKeyBuffer.Destroy()
 
 	secret.DebugWith("Successfully decrypted long-term private key",
 		slog.String("vault_name", v.Name),
 		slog.String("unlocker_type", unlocker.GetType()),
-		slog.Int("decrypted_length", len(ltPrivKeyData)),
+		slog.Int("decrypted_length", ltPrivKeyBuffer.Size()),
 	)
 
 	// Parse long-term private key
 	secret.Debug("Parsing long-term private key", "vault_name", v.Name)
-	ltIdentity, err := age.ParseX25519Identity(string(ltPrivKeyData))
+	ltIdentity, err := age.ParseX25519Identity(ltPrivKeyBuffer.String())
 	if err != nil {
 		secret.Debug("Failed to parse long-term private key", "error", err, "vault_name", v.Name)