`_save_auth_store`, `_save_qwen_cli_tokens`, and `_write_shared_nous_state`
all created the temp file via `Path.open('w')` / `Path.write_text` and only
tightened permissions to 0o600 afterward. Between create and chmod the file
existed at the process umask (commonly 0o644 = world-readable on multi-user
hosts), briefly exposing OAuth access/refresh tokens for Nous, Codex,
Copilot, Claude, Qwen, Gemini, and every other native OAuth provider that
flows through auth.json.
Switch all three to `os.open(O_WRONLY|O_CREAT|O_EXCL, 0o600)` + `os.fdopen`
+ `fsync` so the file is atomic at 0o600 on creation. Tighten each parent
directory (`~/.hermes/`, Qwen auth dir, Nous shared auth dir) to 0o700 so
siblings can't traverse to the creds. `_save_auth_store` also gains a
per-process random temp suffix to match `agent/google_oauth.py` (#19673)
and `tools/mcp_oauth.py` (#21148).
Adds `tests/hermes_cli/test_auth_toctou_file_modes.py` asserting final
file mode 0o600 and parent dir mode 0o700 across all three writers, plus
an explicit `os.open(flags, mode)` check on the main auth.json writer
that would fail if anyone reintroduces the `Path.open('w')` pattern.
POSIX-only (mode bits skipped on Windows).
