Fix EncryptWithPassphrase to accept LockedBuffer for data parameter

- Changed EncryptWithPassphrase to accept *memguard.LockedBuffer instead of []byte
- Updated all callers to pass LockedBuffer:
  - CreatePassphraseUnlocker in vault/unlockers.go
  - Keychain unlocker in keychainunlocker.go
  - Tests in passphrase_test.go
- Removed intermediate dataBuffer creation since data is now already protected
- This ensures sensitive data is protected in memory throughout encryption

internal/secret/passphrase_test.go

@@ -76,10 +76,11 @@ func TestPassphraseUnlockerWithRealFS(t *testing.T) {
 
 	// Test encrypting private key with passphrase
 	t.Run("EncryptPrivateKey", func(t *testing.T) {
-		privKeyData := []byte(agePrivateKey)
+		privKeyBuffer := memguard.NewBufferFromBytes([]byte(agePrivateKey))
+		defer privKeyBuffer.Destroy()
 		passphraseBuffer := memguard.NewBufferFromBytes([]byte(testPassphrase))
 		defer passphraseBuffer.Destroy()
-		encryptedPrivKey, err := secret.EncryptWithPassphrase(privKeyData, passphraseBuffer)
+		encryptedPrivKey, err := secret.EncryptWithPassphrase(privKeyBuffer, passphraseBuffer)
 		if err != nil {
 			t.Fatalf("Failed to encrypt private key: %v", err)
 		}