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

`secret rm ..` deleted the whole vault; `secret rm .` and `secret rm ""`
deleted every secret. rm, mv, the version commands, encrypt and decrypt
built paths from the name unchecked; import checked it only after
reading the source file.

Each now calls vault.ValidateSecretName, which wraps the existing rule,
on the name as given, before building any path; MoveSecret checks both
names before switching the current vault. Its error and README.md state
the rule. The test-only copy of the rule in internal/secret is removed.

The regression test runs each rejected command on a copy of two
in-memory vaults and requires the exact error and an unchanged state
directory.

Model: opus-5-5

internal/cli/secrets.go

@@ -603,6 +603,11 @@ func printSecretsTable(
 func (cli *Instance) ImportSecret(
 	cmd *cobra.Command, secretName, sourceFile string, force bool,
 ) error {
+	err := vault.ValidateSecretName(secretName)
+	if err != nil {
+		return err
+	}
+
 	// Get current vault
 	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {
@@ -649,6 +654,11 @@ func (cli *Instance) ImportSecret(
 
 // RemoveSecret removes a secret from the vault
 func (cli *Instance) RemoveSecret(cmd *cobra.Command, secretName string, _ bool) error {
+	err := vault.ValidateSecretName(secretName)
+	if err != nil {
+		return err
+	}
+
 	// Get current vault
 	currentVlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {
@@ -702,13 +712,8 @@ func (cli *Instance) MoveSecret(
 	srcVaultName, srcSecretName, srcQualified := ParseVaultSecretRef(source)
 	destVaultName, destSecretName, destQualified := ParseVaultSecretRef(dest)
 
-	// If neither is qualified, this is a simple within-vault rename
-	if !srcQualified && !destQualified {
-		return cli.moveSecretWithinVault(cmd, srcSecretName, destSecretName, force)
-	}
-
 	// Cross-vault move requires source to be qualified
-	if !srcQualified {
+	if !srcQualified && destQualified {
 		return errCrossVaultSourceUnqualified
 	}
 
@@ -716,31 +721,46 @@ func (cli *Instance) MoveSecret(
 	// Format: "work:secret default" means move to vault "default"
 	// Format: "work:secret default:newname" means move to vault "default"
 	// with a new name
-	if !destQualified {
+	if srcQualified && !destQualified {
 		// Check if dest is actually a vault name
 		vaults, err := vault.ListVaults(cli.fs, cli.stateDir)
 		if err == nil && slices.Contains(vaults, dest) {
 			// dest is a vault name, use source secret name
 			destVaultName = dest
 			destSecretName = srcSecretName
-		}
-
-		// If destVaultName is still empty, dest is a secret name in source vault
-		if destVaultName == "" {
+		} else {
+			// dest is a secret name in source vault
 			destVaultName = srcVaultName
-			destSecretName = dest
 		}
 	}
 
-	// If destination secret name is empty, use source secret name
-	if destSecretName == "" {
+	// If destination secret name is empty, use source secret name. A plain
+	// rename keeps it empty, so that the check below rejects it.
+	if srcQualified && destSecretName == "" {
 		destSecretName = srcSecretName
 	}
 
+	// Check both names, for every form of the move, before selecting a vault
+	// below, so that a rejected move leaves the current vault as it was.
+	err := vault.ValidateSecretName(srcSecretName)
+	if err != nil {
+		return err
+	}
+
+	err = vault.ValidateSecretName(destSecretName)
+	if err != nil {
+		return err
+	}
+
+	// If neither is qualified, this is a simple within-vault rename
+	if !srcQualified && !destQualified {
+		return cli.moveSecretWithinVault(cmd, srcSecretName, destSecretName, force)
+	}
+
 	// Same vault? Use simple rename if possible (optimization)
 	if srcVaultName == destVaultName {
 		// Select the vault and do a simple move
-		err := vault.SelectVault(cli.fs, cli.stateDir, srcVaultName)
+		err = vault.SelectVault(cli.fs, cli.stateDir, srcVaultName)
 		if err != nil {
 			return fmt.Errorf("failed to select vault '%s': %w", srcVaultName, err)
 		}
@@ -753,7 +773,8 @@ func (cli *Instance) MoveSecret(
 		cmd, srcVaultName, srcSecretName, destVaultName, destSecretName, force)
 }
 
-// moveSecretWithinVault handles rename within the current vault
+// moveSecretWithinVault handles rename within the current vault. Its caller,
+// MoveSecret, has already checked both secret names.
 func (cli *Instance) moveSecretWithinVault(
 	cmd *cobra.Command, source, dest string, force bool,
 ) error {
@@ -808,7 +829,8 @@ func (cli *Instance) moveSecretWithinVault(
 	return nil
 }
 
-// moveSecretCrossVault handles moving between different vaults
+// moveSecretCrossVault handles moving between different vaults. Its caller,
+// MoveSecret, has already checked both secret names.
 func (cli *Instance) moveSecretCrossVault(
 	cmd *cobra.Command,
 	srcVaultName, srcSecretName,