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
