Leave no partial unlocker directory when adding an unlocker fails (closes #48)

CreatePGPUnlocker looked up the GPG key's fingerprint, and the keychain
unlocker got the long-term key, only after writing part of the unlocker,
so a failure there left a directory with no metadata. Both now do every
step that can fail before writing anything. `secret unlocker add pgp`
looks the fingerprint up once, for its duplicate check, and passes it to
CreatePGPUnlocker to record. All four unlocker types write their files
through the new secret.WriteDir, which builds a new directory in a
temporary directory, renames it into place when complete and removes it
on a failure. A directory that already exists, as when an unlocker
replaces one of the same name, is written in place and never removed.

Model: opus-5-5

TODO.md

@@ -25,6 +25,17 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: A failed unlocker add no longer leaves a partial unlocker
+  directory (https://git.eeqj.de/sneak/secret/issues/48).
+  `secret unlocker add pgp` resolves the GPG key's fingerprint once, for
+  its duplicate check, and passes it to `CreatePGPUnlocker` to record.
+  `CreatePGPUnlocker` and `CreateKeychainUnlocker` get the long-term key
+  and encrypt everything before writing anything. All four unlocker
+  types write their files through `secret.WriteDir`: a new unlocker is
+  built in a temporary directory, renamed into place when complete and
+  removed on a failure. One added under the directory name of an
+  existing unlocker is still written into that directory in place
+  (https://git.eeqj.de/sneak/secret/issues/71).
 - 2026-10-04: `secret unlocker select` and `secret unlocker remove`
   skip, with the warning `unlocker list` gives, an unlocker directory
   whose metadata file cannot be checked for, read or parsed, instead of
@@ -129,13 +140,10 @@ Bring the repo into policy compliance in one commit:
     - from `init` or `vault create` killed after the passphrase prompt
       but before the unlocker is written, a vault with no unlocker,
       which `vault create` has already made the current vault;
-    - from an unlocker add stopped before its metadata is written, a
-      directory that `unlocker list` warns about and `unlocker rm`
-      removes only by its directory name;
-    - data under a `.tmp-` name in the state directory: a secret or
-      version being added, or the secret, version, unlocker or vault
-      being removed, encrypted keys included. Nothing deletes it; it
-      must be deleted by hand
+    - data under a `.tmp-` name in the state directory: a secret,
+      version or unlocker being added, or the secret, version, unlocker
+      or vault being removed, encrypted keys included. Nothing deletes
+      it; it must be deleted by hand
       (https://git.eeqj.de/sneak/secret/issues/75).
 - 2026-10-03: The checks run before changing a vault now stop with an
   error naming the path and cause when they cannot read what they