Give every new unlocker a directory of its own (closes #71)

A passphrase unlocker added to a vault that had one, and a PGP, keychain
or Secure Enclave unlocker added on the same day as another of its type,
were written into the existing unlocker's directory file by file, so a
crash part-way left a current unlocker whose files did not belong
together.

Unlocker directories, keychain items and Secure Enclave keys are now
named with the time to the nanosecond, and secret.WriteDir refuses a
directory that exists. Adding a passphrase unlocker writes the new one,
points current-unlocker at it, and only then removes the vault's other
passphrase unlockers.

Model: opus-5-5

internal/secret/seunlocker_darwin.go

@@ -193,14 +193,14 @@ func generateSEKeyLabel(vaultName string) (string, error) {
 		return "", fmt.Errorf("failed to get hostname: %w", err)
 	}
 
-	enrollmentDate := time.Now().UTC().Format("2006-01-02")
+	enrollmentTime := time.Now().UTC().Format(UnlockerTimeFormat)
 
 	return fmt.Sprintf(
 		"%s.%s-%s-%s",
 		seKeyLabelPrefix,
 		vaultName,
 		hostname,
-		enrollmentDate,
+		enrollmentTime,
 	), nil
 }