Lock the state directory and write vault files atomically (closes #34)

Each command that changes the state directory holds one lock: flock(2)
on `lock` in the state directory, dropped by the kernel if the process
dies, or a process-wide mutex on the in-memory test filesystem. It
covers the state directory, not each vault, because `currentvault`,
`vault create` and cross-vault moves span vaults, and a lock file in a
vault would be deleted by `vault remove` under a waiting command.

Files go through `secret.WriteFileAtomic`; versions, new secrets and
cross-vault copies are built in a temporary directory and renamed into
place; removals rename out of the way first. An unlocker added under
an existing unlocker's directory name is still rewritten file by file:
#71.

Model: opus-5-5

TODO.md

@@ -25,6 +25,21 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: Commands that change the state directory hold one lock
+  (`flock` on `lock` in the state directory; a mutex on the in-memory
+  test filesystem), so concurrent commands no longer lose versions or
+  race on the current pointers. Every file is written through
+  `secret.WriteFileAtomic` (temporary file, sync, rename); 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 an interrupted command leaves nothing
+  half-written, with one exception: an unlocker added under the
+  directory name of an existing one is rewritten in place, file by
+  file, and a crash part-way leaves it unable to open the vault. That
+  happens to a passphrase unlocker added to a vault that has one, and
+  to a PGP, keychain or Secure Enclave unlocker added on the same host
+  and day as another of its type
+  (https://git.eeqj.de/sneak/secret/issues/71).
 - 2026-10-02: A plain `docker build .` builds again: the size tests
   skip a case that needs more locked memory than the process can
   lock, and run every case under `script/cibuild`. The image stamps the
@@ -94,8 +109,6 @@ Bring the repo into policy compliance in one commit:
       pgpunlocker.go:256, version.go:155); age secret key held in a
       plain string in cli/crypto.go:86,91,113; private keys exposed via
       buffer.Bytes() to GPGEncryptFunc and EncryptWithPassphrase.
-    - Race conditions: no file locking in vault/secrets.go:142-176;
-      non-atomic writes can leave the vault inconsistent.
     - Input validation: dots in secret names risk path traversal
       (vault/secrets.go:75-99); no maximum secret size (DoS).
     - Timing attacks: bytes.Equal passphrase compare (cli/init.go: