Create a vault whole in a temporary directory, then select it (closes #105)

vault.CreateVault takes the unlocker passphrase and writes the vault
directory, its metadata, long-term public key and passphrase unlocker
into a temporary directory, renames that into vaults.d once complete,
and only then makes the vault current. secret init and secret vault
create call it once instead of adding the unlocker afterwards, so a
kill part-way leaves either no vault, whose temporary directory the
next command that takes the lock deletes, or a complete one. A test
records the state directory before every change the call makes and
checks each state, and the command run again from it.

Model: opus-5-5

TODO.md

@@ -18,6 +18,19 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-04: `secret init` and `secret vault create` create a vault whole or
+  not at all (https://git.eeqj.de/sneak/secret/issues/105).
+  `vault.CreateVault` now takes the unlocker passphrase too, writes the vault
+  directory with its metadata, long-term public key and passphrase unlocker,
+  `longterm.age` included, into a temporary directory, renames that into
+  `vaults.d` once it is complete, and only then makes the vault current.
+  Before, either command killed after the passphrase prompt but before the
+  unlocker was written left a vault with no unlocker, which `vault create` had
+  already made current and which neither command would create again. Killed
+  part-way now, it leaves no vault, and the next command that takes the lock
+  deletes the temporary directory; or, killed between the rename and making
+  the vault current, a complete vault that is not current, which
+  `secret vault select` makes current.
 - 2026-10-04: A failed `secret unlocker add keychain` or
   `secret unlocker add secure-enclave` no longer leaves its keychain item or
   Secure Enclave key behind (https://git.eeqj.de/sneak/secret/issues/89).
@@ -258,11 +271,7 @@ https://git.eeqj.de/sneak/secret/milestone/12
   `current-unlocker` never go missing. New versions, new secrets and
   cross-vault copies are built in a temporary directory and renamed
   into place, and removals rename out of the way first, so a version
-  or secret is never half-added and never half-removed. An
-  interrupted command can still leave, 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.
+  or secret is never half-added and never half-removed.
 - 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
   inspect, instead of reading the failure as "nothing there": the