Routes the two credential-writing call sites (xfetch session sync, no-Docker XHS cookie fallback) through the same atomic os.open(..., 0o600) pattern Config.save() already uses, closing a TOCTOU window where credential files were briefly world-readable (CWE-732). Also hardens _sync_bird_env() with shlex.quote against a shell-injection breakout in the sourceable env file — verified exploitable on the pre-fix code. Maintainer follow-up: scoped the injection-probe test markers to tmp_path so they can't poison reruns. 107 tests pass.
@aaronjmars