[Feature] Add Github Copilot subscription support

[rothnic]

Description

I use opencode, which provides the only option I'm aware of at the moment to leverage the copilot subscription with an alternative cli and sdk. I saw that there was a recent feature implemented for github copilot, but after some looking into it, it isn't quite the same. GIthub has some pretty confusing docs, so this is easy to get confused over. Github Models is kind of like OpenRouter. Github Copilot is more like Claude Code or Codex.

Github Models provides some small amount of free usage each day, but would be far, far more expensive to use directly as part of a coding agent, compared to using Github Copilot due to the sheer amount of tokens processed and the much more user-friendly billing model for how Github Copilot subscriptions work oriented around prompts rather than token-based billing. A single prompt can result in many many tool calls and api calls, and there is no risk that you are going to accidentally blow up the context window and get charged heavily for it.

Use Case

• Using the pro+ subscription allows me near unlimited free use of gpt-5-mini and grok code fast 4. This is one of the most compelling aspects of the subscription because it enables you to do as much subtasking as you want for routine, small or concise tasks (cleaning up docs, summarizing, git workflows, etc)
• I also get ~1500 premium requests per month, which at $40 per month seems to be on par in terms of usage compared to other $200/month subscriptions (e.g., claude code, codex, etc)

Proposed Solution

Implement the same proposed authentication approach outlined in the original feature request #67, but was marked solved with #108. Github Models isn't the same thing as Github Copilot.

Alternatives Considered

No alternative i'm aware of

Additional Context

• I have searched existing issues to ensure this is not a duplicate
• This feature aligns with the project's goals (local-first AI assistance)

Implementation Notes (optional)

Just, overall the opencode repository contains everything needed to make this work

[will-lamerton]

Hey! Thanks for this. I've done some research and unlike Anthropic and OpenAI, GitHub Copilot is a lot more permissive with this. I don't have a co-pilot subscription myself but happy to work with someone that does to get this right and test.