Reject invalid secret names before any command builds a path (closes #33)

`secret rm ..` resolved to the vault directory and deleted the whole
vault; `secret rm .` and `secret rm ""` deleted every secret. rm, mv and
import built paths from the name without checking it, and so did the
version commands, encrypt and decrypt.

vault.ValidateSecretName wraps the existing name rule and returns
ErrInvalidSecretName. Each of those commands calls it on the name as
given, both names for a move, before building any path. AddSecret and
GetSecretVersion use it too, so the rule has one implementation.

The regression test snapshots every file under the state directory of
two in-memory vaults and requires it unchanged after each rejected
command.

Model: opus-5-5

internal/vault/secrets.go

@@ -110,6 +110,20 @@ func isValidSecretName(name string) bool {
 	return matched
 }
 
+// ValidateSecretName returns an error wrapping ErrInvalidSecretName when
+// name is not a valid secret name. Call it on the name exactly as the user
+// gave it, before building any path from it.
+func ValidateSecretName(name string) error {
+	if !isValidSecretName(name) {
+		return fmt.Errorf(
+			"%w '%s': must match pattern [a-z0-9.\\-_/]+",
+			ErrInvalidSecretName, name,
+		)
+	}
+
+	return nil
+}
+
 // AddSecret adds a secret to this vault
 func (v *Vault) AddSecret(name string, value *memguard.LockedBuffer, force bool) error {
 	if value == nil {
@@ -124,13 +138,11 @@ func (v *Vault) AddSecret(name string, value *memguard.LockedBuffer, force bool)
 	)
 
 	// Validate secret name
-	if !isValidSecretName(name) {
+	err := ValidateSecretName(name)
+	if err != nil {
 		secret.Debug("Invalid secret name provided", "secret_name", name)
 
-		return fmt.Errorf(
-			"%w '%s': must match pattern [a-z0-9.\\-_/]+",
-			ErrInvalidSecretName, name,
-		)
+		return err
 	}
 
 	secret.Debug("Secret name validation passed", "secret_name", name)
@@ -628,13 +640,11 @@ func (v *Vault) updatePreviousVersion(
 // version exist, and resolves an empty version to the current one.
 func (v *Vault) resolveSecretVersion(name, version string) (string, error) {
 	// Validate secret name to prevent path traversal
-	if !isValidSecretName(name) {
+	err := ValidateSecretName(name)
+	if err != nil {
 		secret.Debug("Invalid secret name provided", "secret_name", name)
 
-		return "", fmt.Errorf(
-			"%w '%s': must match pattern [a-z0-9.\\-_/]+",
-			ErrInvalidSecretName, name,
-		)
+		return "", err
 	}
 
 	// Get vault directory