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

TODO.md

@@ -25,6 +25,11 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: A PGP unlocker whose metadata has no usable GPG key ID
+  no longer panics: `GetID()` warns with the unlocker's directory and
+  returns `pgp-unknown`. `ListUnlockers` skips, with a warning, an
+  unlocker whose metadata file cannot be read or parsed instead of
+  failing, so `secret unlocker list` still lists the others.
 - 2026-10-02: A plain `docker build .` builds again: the size tests
   skip a case that needs more locked memory than the process can
   lock, and run every case under `script/cibuild`. The image stamps the
@@ -101,8 +106,6 @@ Bring the repo into policy compliance in one commit:
     - Timing attacks: bytes.Equal passphrase compare (cli/init.go:
       209-216); non-constant-time public key compare (vault.go:95-100).
 - High priority:
-    - Return errors instead of panicking on corrupted metadata
-      (pgpunlocker.go:116, keychainunlocker.go:141).
     - Secure temporary file handling and cleanup.
     - Print cobra usage only for argument errors, not internal
       failures.