New built-in image_gen backend at plugins/image_gen/openai-codex/ that
exposes the same gpt-image-2 low/medium/high tier catalog as the
existing 'openai' plugin, but routes generation through the ChatGPT/
Codex Responses image_generation tool path. Available whenever the user
has Codex OAuth signed in; no OPENAI_API_KEY required.
The two plugins are independent — users select between them via
'hermes tools' → Image Generation, and image_gen.provider in
config.yaml. The existing 'openai' (API-key) plugin is unchanged.
Reuses _read_codex_access_token() and _codex_cloudflare_headers() from
agent.auxiliary_client so token expiry / cred-pool / Cloudflare
originator handling stays in one place.
Inspired by #14047 by @Hygaard, but re-implemented as a separate
plugin instead of an in-place fork of the openai plugin.
Closes#11195
