fix: use vault derivation index in getLongTermPrivateKey instead of hardcoded 0

Previously, getLongTermPrivateKey() always used derivation index 0 when
deriving the long-term key from a mnemonic. This caused wrong key
derivation for vaults with index > 0 (second+ vault from same mnemonic),
leading to silent data corruption in keychain unlocker creation.

Now reads the vault's actual DerivationIndex from vault-metadata.json.

internal/secret/keychainunlocker.go

@@ -251,8 +251,25 @@ func getLongTermPrivateKey(fs afero.Fs, vault VaultInterface) (*memguard.LockedB
 	// Check if mnemonic is available in environment variable
 	envMnemonic := os.Getenv(EnvMnemonic)
 	if envMnemonic != "" {
-		// Use mnemonic directly to derive long-term key
+		// Read vault metadata to get the correct derivation index
-		ltIdentity, err := agehd.DeriveIdentity(envMnemonic, 0)
+		vaultDir, err := vault.GetDirectory()
+		if err != nil {
+			return nil, fmt.Errorf("failed to get vault directory: %w", err)
+		}
+
+		metadataPath := filepath.Join(vaultDir, "vault-metadata.json")
+		metadataBytes, err := afero.ReadFile(fs, metadataPath)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read vault metadata: %w", err)
+		}
+
+		var metadata VaultMetadata
+		if err := json.Unmarshal(metadataBytes, &metadata); err != nil {
+			return nil, fmt.Errorf("failed to parse vault metadata: %w", err)
+		}
+
+		// Use mnemonic with the vault's actual derivation index
+		ltIdentity, err := agehd.DeriveIdentity(envMnemonic, metadata.DerivationIndex)
 		if err != nil {
 			return nil, fmt.Errorf("failed to derive long-term key from mnemonic: %w", err)
 		}