Keep the keychain unlocker passphrase in locked memory (closes #36)

The passphrase protecting the keychain unlocker's age key was a plain
string passed through encoding/json, leaving copies in ordinary memory
when an unlocker was created and each time one was used.

It is now generated into a locked buffer, and KeychainData, moved to
keychaindata.go, which is not darwin-only so its tests run on Linux,
writes and reads the keychain JSON itself: encode copies the parts
straight into a locked buffer, and decodeKeychainData takes the
passphrase from a json.RawMessage that it wipes. The JSON field names
are unchanged. keychainunlocker.go only calls this code and stores the
item from the locked buffer without a string copy.

Model: opus-5-5

TODO.md

@@ -25,6 +25,11 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 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
+  `internal/secret/keychaindata.go` (tested on Linux) without
+  `encoding/json` holding it; the JSON field names are unchanged.
 - 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
@@ -88,12 +93,11 @@ Bring the repo into policy compliance in one commit:
     - Command injection: GPG key IDs passed unescaped to exec.Command
       (pgpunlocker.go:323-327); data.String() passed unescaped to the
       security command (keychainunlocker.go:472-476).
-    - Memory security: KeychainData stores AgePrivKeyPassphrase as a
-      plain string (keychainunlocker.go:342,393-396); age identity
-      .String() creates unprotected copies (keychainunlocker.go:356,
-      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.
+    - Memory security: age identity .String() creates unprotected
+      copies (keychainunlocker.go:356, 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