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. Left for later: replacing
an unlocker (#71) and deleting
what an interrupted command leaves under a `.tmp-` name
(#75).

Model: opus-5-5

TODO.md

@@ -25,6 +25,34 @@ 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), so no file
+  is ever half-written and `current`, `currentvault` and
+  `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:
+    - a broken unlocker, when it was replacing one: an unlocker added
+      under the directory name of an existing one is rewritten file by
+      file. 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);
+    - from `vault create` stopped at the passphrase prompt, a new vault
+      with no unlocker that is already the current vault; from `init`
+      stopped there, the default vault with no unlocker;
+    - from an unlocker add stopped before its metadata is written, a
+      directory that `unlocker list` warns about and `unlocker rm`
+      cannot remove;
+    - 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
+      (https://git.eeqj.de/sneak/secret/issues/75).
 - 2026-10-03: The keychain unlocker's age key passphrase stays in
   locked memory: it is generated into a locked buffer, and the
   keychain JSON is written and read by `KeychainData` code in
@@ -98,8 +126,6 @@ Bring the repo into policy compliance in one commit:
       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: