Stop vault safety checks from reading unreadable state as empty (closes #51)

Adding a PGP unlocker checked unlockers.d for a duplicate and, when the
directory could not be read, reported no duplicate and went on. The
check now returns an error naming the directory and cause, and the add
stops; a duplicate found is still reported as one.

The same flaw guarded removing the last unlocker and removing a vault
(an unreadable secrets directory counted as no secrets) and vault
import (an unreadable pub.age counted as no long-term key). Those now
stop with an error too.

`unlocker list` keeps skipping entries it cannot read; a comment at the
duplicate check says why the two differ.

Model: opus-5-5

internal/cli/unlockers.go

@@ -49,7 +49,6 @@ var (
 		"is already added as an unlocker")
 	errUnsupportedUnlockerType = errors.New("unsupported unlocker type")
 	errLastUnlocker            = errors.New("refusing to remove last unlocker")
-	errUnlockerExists          = errors.New("unlocker already exists")
 )
 
 // UnlockerInfo represents unlocker information for display
@@ -691,8 +690,15 @@ func (cli *Instance) addPGPUnlocker(cmd *cobra.Command) error {
 	// Check if this GPG key is already added
 	expectedID := "pgp-" + fingerprint
 
-	err = cli.checkUnlockerExists(vlt, expectedID)
+	exists, err := cli.checkUnlockerExists(vlt, expectedID)
 	if err != nil {
+		return fmt.Errorf(
+			"could not check whether GPG key %s is already an unlocker: %w",
+			gpgKeyID, err,
+		)
+	}
+
+	if exists {
 		return fmt.Errorf("GPG key %s %w", gpgKeyID, errGPGKeyAlreadyUnlocker)
 	}
 
@@ -772,44 +778,38 @@ func (cli *Instance) UnlockerSelect(unlockerID string) error {
 	return vlt.SelectUnlocker(unlockerID)
 }
 
-// checkUnlockerExists checks if an unlocker with the given ID exists
-func (cli *Instance) checkUnlockerExists(vlt *vault.Vault, unlockerID string) error {
-	// Get the list of unlockers and check if any match the ID
-	unlockers, err := vlt.ListUnlockers()
-	if err != nil {
-		secret.Warn("Could not list unlockers during duplicate check", "error", err)
-
-		return nil // If we can't list unlockers, assume it doesn't exist
-	}
-
-	// Get vault directory to construct unlocker instances
+// checkUnlockerExists reports whether the vault already has an unlocker
+// with the given ID. It returns an error, and no answer, when unlockers.d
+// cannot be read; the caller must then not create the unlocker.
+func (cli *Instance) checkUnlockerExists(
+	vlt *vault.Vault, unlockerID string,
+) (bool, error) {
 	vaultDir, err := vlt.GetDirectory()
 	if err != nil {
-		secret.Warn("Could not get vault directory during duplicate check",
-			"error", err)
-
-		return nil
+		return false, fmt.Errorf("failed to get vault directory: %w", err)
 	}
 
-	// Check each unlocker's ID
 	unlockersDir := filepath.Join(vaultDir, "unlockers.d")
 
+	unlockers, err := vlt.ListUnlockers()
+	if err != nil {
+		return false, fmt.Errorf(
+			"failed to list unlockers in %s: %w", unlockersDir, err,
+		)
+	}
+
 	for _, metadata := range unlockers {
 		// Construct the unlocker matching this metadata to get its ID
 		id, err := findUnlockerIDByMetadata(cli.fs, unlockersDir, metadata, true)
 		if err != nil {
-			secret.Warn(
-				"Could not read unlockers directory during duplicate check, "+
-					"skipping unlocker",
-				"unlockers_dir", unlockersDir, "error", err)
-
-			continue
+			// Unlike `unlocker list`, never skip here: a skipped entry may be the duplicate.
+			return false, err
 		}
 
 		if id != "" && id == unlockerID {
-			return errUnlockerExists
+			return true, nil
 		}
 	}
 
-	return nil
+	return false, nil
 }