fix(opencode): use public Copilot host for github.com - #51895

The following comment was made by an LLM, it may be inaccurate:

github.com OAuth accounts are sent to the enterprise-remapped Copilot host instead of api.githubcopilot.com.

Head 633fbe9c3fb448e7de17e9e70aabb6b88eca9681 is unchanged from the tested tip.

Fork leaf kvnloo#193 recorded fail→pass on packages/opencode/test/plugin/github-copilot-models.test.ts (8 pass, 1 fail with the assertion removed, 8 pass after restore).

That locks the public host for github.com accounts. It does not cover enterprise hosts.

@kvnloo Thanks for the review and the fork-based verification! Acknowledged the issue: github.com OAuth accounts were being incorrectly routed to the enterprise Copilot host. I'll keep the existing enterprise-host behavior as-is (head unchanged) and focus the fix strictly on the public host routing for github.com accounts. Happy to discuss the fork/branch handling approach — if there's a preferred way to structure this (e.g., separate commits, test coverage for enterprise hosts), let me know.