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 is unreadable or not JSON, as it already did for a missing one.

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

Model: opus-5-5

internal/vault/unlockers.go

@@ -247,16 +247,20 @@ 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)