feat: Sign in with ChatGPT

[neominik]

Adds support for signing in with ChatGPT (same as in codex) through opencode auth login. Unfortunately, I cannot test if it actually works because support for enterprise accounts is "coming soon".

If anyone with a Plus, Pro, or Team ChatGPT account could test this via bun run dev -- auth login, that would be awesome.

[wizardlyluke]

I just tried this out with my Pro account and I get a missing organization_id error.

◒  Waiting for authorizationAPI key exchange failed: 401 Unauthorized {
  "error": {
    "message": "Invalid ID token: missing organization_id",
    "type": "invalid_request_error",
    "param": null,
    "code": "invalid_subject_token"
  }

[neominik]

Thanks for trying! I aligned the implementation more closely with what codex does. Not sure if it will fix your issue. If not, I'll convert to draft and park this until enterprise accounts are supported unless someone else want's to give it a shot.

[wizardlyluke]

Alright I seem to get a different error now. This login works with codex so I'm guessing something else is up. I was thinking it wasn't passing the organization ID but I couldn't get it to work when doing that either, or I'm just not doing it right.
■ Login incomplete — please finish your OpenAI organization/project setup in the browser