ChatGPT sign-in through the open-source route

A browser sign-in for --provider codex now uses OpenAI’s open-source Sign in
with ChatGPT by default. Yolop registers its own client for your account on the
first sign-in and runs turns on the public Responses API, still billed to your
ChatGPT plan. It is a preview route, so a few request options (output token
cap, temperature, speed) do not apply.
To keep the Codex login, set the route and sign in again:
yolop config set chatgpt_sign_in codexYOLOP_CHATGPT_SIGN_IN=codex does the same. A device login uses the Codex
route unless open-source is set.
See Providers and models, #806 and #808.