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

internal/cli/unlockers.go

@@ -349,6 +349,10 @@ func unlockerIDFromDir(
 // itself cannot be read. Callers must distinguish the two: an unreadable
 // directory means the unlocker's real ID is unknowable, so the entry has
 // to be skipped rather than reported under a synthesized ID.
+//
+// A metadata file that cannot be read or parsed is skipped without a
+// warning: every caller gets metadata from vault.ListUnlockers first,
+// which has already warned about that directory.
 func findUnlockerIDByMetadata(
 	fs afero.Fs, unlockersDir string, metadata secret.UnlockerMetadata,
 	includeSecureEnclave bool,
@@ -371,9 +375,6 @@ func findUnlockerIDByMetadata(
 		// Check if this is the right unlocker by comparing metadata
 		metadataBytes, err := afero.ReadFile(fs, metadataPath)
 		if err != nil {
-			secret.Warn("Could not read unlocker metadata file",
-				"path", metadataPath, "error", err)
-
 			continue
 		}
 
@@ -381,9 +382,6 @@ func findUnlockerIDByMetadata(
 
 		err = json.Unmarshal(metadataBytes, &diskMetadata)
 		if err != nil {
-			secret.Warn("Could not parse unlocker metadata file",
-				"path", metadataPath, "error", err)
-
 			continue
 		}