Gate round-1 follow-ups on the #121486 fix:
- auxiliary_client: inline the pool route lookup (no dead try/except or
fallbacks; HERMES_CODEX_BASE_URL short-circuits once) and read auth.json
directly when the pool yields no token (no second uncached pool load,
no re-select race pairing a new pool key with chatgpt.com).
- image plugin: _read_codex_credential() is the single source for both
is_available() and generate(); _post_image_request requires base_url.
- auth_codex: drop the unused _pool_codex_access_token wrapper; the route
helper's error fallback reads the profile-scoped override, not the raw
process env.
- model setup flow: the confirm guards get the resolved Codex base, not
the chatgpt.com constant.
- cli_model_switch_mixin: self.base_url is always set.
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