Keep unlocker list working when unlocker metadata is corrupt (closes #42)

PGPUnlocker.GetID() panicked when its metadata could not be read or
parsed, which took down `secret unlocker list` for every unlocker. It
now warns with the unlocker's directory and returns `pgp-unknown`;
metadata with an empty GPG key ID counts as corrupt too.
ListUnlockers now skips, with a warning, an unlocker whose metadata
file cannot be checked for, read or parsed, as it already did for a
missing one. The listing's ID lookup skips such a directory without
warning again.

This is the first half of the issue only. Passing the mnemonic in
memory moved to #60.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/vault/unlockers.go

@@ -233,9 +233,10 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 
 			exists, err := afero.Exists(v.fs, metadataPath)
 			if err != nil {
-				return nil, fmt.Errorf(
-					"failed to check if metadata exists for unlocker %s: %w",
-					file.Name(), err)
+				secret.Warn("Skipping unlocker directory whose metadata file cannot be checked",
+					"directory", file.Name(), "error", err)
+
+				continue
 			}
 
 			if !exists {
@@ -247,16 +248,20 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 
 			metadataBytes, err := afero.ReadFile(v.fs, metadataPath)
 			if err != nil {
-				return nil, fmt.Errorf(
-					"failed to read metadata for unlocker %s: %w", file.Name(), err)
+				secret.Warn("Skipping unlocker directory with unreadable metadata file",
+					"directory", file.Name(), "error", err)
+
+				continue
 			}
 
 			var metadata UnlockerMetadata
 
 			err = json.Unmarshal(metadataBytes, &metadata)
 			if err != nil {
-				return nil, fmt.Errorf(
-					"failed to parse metadata for unlocker %s: %w", file.Name(), err)
+				secret.Warn("Skipping unlocker directory with corrupt metadata file",
+					"directory", file.Name(), "error", err)
+
+				continue
 			}
 
 			unlockers = append(unlockers, metadata)