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/secret/pgpunlocker.go

@@ -155,14 +155,18 @@ func (p *PGPUnlocker) GetDirectory() string {
 	return p.Directory
 }
 
-// GetID implements Unlocker interface - generates ID from GPG key ID
+// GetID implements Unlocker interface - generates ID from GPG key ID.
+// If the metadata has no usable GPG key ID, it warns with the unlocker's
+// directory and returns "pgp-unknown", so listing the other unlockers
+// still works.
 func (p *PGPUnlocker) GetID() string {
 	// Generate ID using GPG key ID: pgp-<keyid>
 	gpgKeyID, err := p.GetGPGKeyID()
 	if err != nil {
-		// The vault metadata is corrupt - this is a fatal error
-		// We cannot continue with a fallback ID as that would mask data corruption
-		panic(fmt.Sprintf("PGP unlocker metadata is corrupt or missing GPG key ID: %v", err))
+		Warn("PGP unlocker metadata is corrupt or missing its GPG key ID",
+			"directory", p.Directory, "error", err)
+
+		return "pgp-unknown"
 	}
 
 	return "pgp-" + gpgKeyID
@@ -197,6 +201,10 @@ func (p *PGPUnlocker) GetGPGKeyID() (string, error) {
 		return "", fmt.Errorf("failed to parse PGP metadata: %w", err)
 	}
 
+	if pgpMetadata.GPGKeyID == "" {
+		return "", fmt.Errorf("PGP metadata: %w", errGPGKeyIDEmpty)
+	}
+
 	return pgpMetadata.GPGKeyID, nil
 }