Skip unlocker directories with missing metadata instead of failing

When an unlocker directory exists but is missing unlocker-metadata.json,
log a debug warning and skip it instead of returning a hard error that
crashes the entire 'unlocker ls' command.

Closes #1

internal/vault/unlockers.go

@@ -213,7 +213,9 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 				return nil, fmt.Errorf("failed to check if metadata exists for unlocker %s: %w", file.Name(), err)
 			}
 			if !exists {
-				return nil, fmt.Errorf("unlocker directory %s is missing metadata file", file.Name())
+				secret.Debug("Skipping unlocker directory with missing metadata file", "directory", file.Name())
+
+				continue
 			}
 
 			metadataBytes, err := afero.ReadFile(v.fs, metadataPath)