Make an unlocker's ID the name of its directory (closes #98)

Keychain and Secure Enclave unlocker IDs were the creation time to the
minute plus the host name, and passphrase unlocker IDs the time to the
minute, so two created within one minute shared an ID, and `unlocker
select`, `unlocker remove` and the selection after `unlocker add` acted on
the older one. Every unlocker's ID is now its directory name, unique in
its vault. `vault.ListUnlockers` returns each unlocker's metadata keyed by
that name, so `unlocker list` and shell completion no longer find IDs by
matching metadata. PGP unlocker IDs were `pgp-<fingerprint>`; a second
PGP unlocker for one key is refused by comparing fingerprints in metadata.

Model: opus-5-5

TODO.md

@@ -18,6 +18,24 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-04: An unlocker's ID is the name of its directory in `unlockers.d`,
+  so no two unlockers of a vault share one
+  (https://git.eeqj.de/sneak/secret/issues/98). Before, a keychain or Secure
+  Enclave unlocker's ID was its creation time to the minute and the host name,
+  and a passphrase unlocker's the time to the minute, so two created within a
+  minute shared an ID, and `unlocker select`, `unlocker remove` and the
+  selection `unlocker add` makes acted on the older one. A PGP unlocker's ID
+  was `pgp-` and its key's fingerprint; a second PGP unlocker for a key is
+  still refused, now by comparing the fingerprint in the other unlockers'
+  metadata. `unlocker list` and the shell completion of `unlocker select` and
+  `unlocker remove` take each ID from the directory the unlocker was read
+  from, no longer by matching metadata, so two unlockers with the same
+  metadata are listed apart; an unlocker of an unknown type is listed under
+  its directory name, and completion now offers Secure Enclave unlockers too.
+  The keychain and Secure Enclave code was type-checked by
+  `script/lint-darwin`, never run; a test on Linux lists, completes, selects
+  and removes each of two passphrase unlockers with the same metadata by its
+  own ID.
 - 2026-10-04: README's Storage Architecture, `secret version promote`,
   Technical Details and Testing text matches the code
   (https://git.eeqj.de/sneak/secret/issues/102). `current` and