Keep secret get values in locked memory (closes #37)

Vault.GetSecret and Vault.GetSecretVersion return the decrypted value
as a *memguard.LockedBuffer instead of copying it into an ordinary
[]byte that nothing wiped. Every caller destroys the buffer, and
`secret get` writes its bytes straight to stdout, still with no
trailing newline. Instance.Print, which formatted through fmt and had
no other callers, is removed, and so is a debug log line in
`get --version` that held the plaintext value.

Model: opus-5-5

internal/vault/secrets.go

@@ -301,8 +301,9 @@ func updateVersionMetadata(
 	return nil
 }
 
-// GetSecret retrieves the current version of a secret from this vault
-func (v *Vault) GetSecret(name string) ([]byte, error) {
+// GetSecret retrieves the current version of a secret from this vault.
+// The caller must destroy the returned buffer.
+func (v *Vault) GetSecret(name string) (*memguard.LockedBuffer, error) {
 	secret.DebugWith("Getting secret from vault",
 		slog.String("vault_name", v.Name),
 		slog.String("secret_name", name),
@@ -326,7 +327,10 @@ func (v *Vault) GetSecret(name string) ([]byte, error) {
 
 // GetSecretVersion retrieves a specific version of a secret. The version
 // must be one of the secret's versions; GetSecret gets the current one.
-func (v *Vault) GetSecretVersion(name string, version string) ([]byte, error) {
+// The caller must destroy the returned buffer.
+func (v *Vault) GetSecretVersion(
+	name string, version string,
+) (*memguard.LockedBuffer, error) {
 	secret.DebugWith("Getting secret version from vault",
 		slog.String("vault_name", v.Name),
 		slog.String("secret_name", name),
@@ -372,26 +376,14 @@ func (v *Vault) GetSecretVersion(name string, version string) ([]byte, error) {
 		return nil, fmt.Errorf("failed to decrypt version: %w", err)
 	}
 
-	// Create a copy to return since the buffer will be destroyed
-	result := make([]byte, decryptedValue.Size())
-	copy(result, decryptedValue.Bytes())
-	decryptedValue.Destroy()
-
 	secret.DebugWith("Successfully decrypted secret version",
 		slog.String("secret_name", name),
 		slog.String("version", version),
 		slog.String("vault_name", v.Name),
-		slog.Int("decrypted_length", len(result)),
+		slog.Int("decrypted_length", decryptedValue.Size()),
 	)
 
-	// Debug: Log metadata about the decrypted value without exposing the actual secret
-	secret.Debug("Vault secret decryption debug info",
-		"secret_name", name,
-		"version", version,
-		"decrypted_value_length", len(result),
-		"is_empty", len(result) == 0)
-
-	return result, nil
+	return decryptedValue, nil
 }
 
 // UnlockVault unlocks the vault and returns the long-term private key