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

internal/vault/unlockers.go

@@ -188,8 +188,9 @@ func (v *Vault) findUnlockerByID(
 	return nil, skippedDirPath, nil
 }
 
-// ListUnlockers returns a list of available unlockers for this vault
-func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
+// ListUnlockers returns the metadata of each unlocker of this vault, keyed
+// by the unlocker's ID, the name of its directory in unlockers.d
+func (v *Vault) ListUnlockers() (map[string]UnlockerMetadata, error) {
 	vaultDir, err := v.GetDirectory()
 	if err != nil {
 		return nil, err
@@ -204,7 +205,7 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 	}
 
 	if !exists {
-		return []UnlockerMetadata{}, nil
+		return map[string]UnlockerMetadata{}, nil
 	}
 
 	// List directories in unlockers.d
@@ -213,7 +214,7 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 		return nil, fmt.Errorf("failed to read unlockers directory: %w", err)
 	}
 
-	var unlockers []UnlockerMetadata
+	unlockers := map[string]UnlockerMetadata{}
 
 	for _, file := range files {
 		if !file.IsDir() {
@@ -222,7 +223,7 @@ func (v *Vault) ListUnlockers() ([]UnlockerMetadata, error) {
 
 		metadata, ok := v.readUnlockerMetadataOrWarn(unlockersDir, file.Name())
 		if ok {
-			unlockers = append(unlockers, metadata)
+			unlockers[file.Name()] = metadata
 		}
 	}
 
@@ -453,8 +454,7 @@ func writePassphraseUnlocker(
 		return nil, err
 	}
 
-	// Select the new unlocker by its directory, not by its ID: an old
-	// passphrase unlocker created in the same minute has the same ID.
+	// Make the new unlocker the current one
 	currentUnlockerPath := filepath.Join(vaultDir, "current-unlocker")
 
 	err = secret.WriteFileAtomic(fs, currentUnlockerPath,