GITHUB_TOKEN incorrectly activates GitHub Copilot provider

Description

When GITHUB_TOKEN is set for GitHub tooling, OpenCode also treats github-copilot as connected through generic environment authentication, even when no Copilot OAuth credential exists. opencode auth list consequently reports GitHub Copilot under Environment.

This bypasses Copilot-specific OAuth, model-discovery, and request-routing logic. Without the endpoint metadata returned by Copilot model discovery, gpt-5.6-sol is sent to /chat/completions, resulting in request failure every time.

The minimal expected behavior is to ignore GITHUB_TOKEN when automatically activating the github-copilot provider and require the existing Copilot OAuth flow. Generic environment authentication for other providers should remain unchanged.

Plugins

None; this concerns the built-in GitHub Copilot provider.

OpenCode version

dev at e00890c672 (1.18.21)

Steps to reproduce

1. Use a clean OpenCode data directory with no stored github-copilot credential.
2. Set a non-empty GITHUB_TOKEN.
3. Run opencode auth list.
4. Observe that GitHub Copilot is reported under Environment despite no Copilot OAuth login.
5. Select a newer Copilot model such as github-copilot/gpt-5.6-sol.
6. Observe that OpenCode uses the generic environment-token provider path instead of Copilot OAuth/model discovery, resulting in gpt-5.6-sol being sent to /chat/completions and failing.

Screenshot and/or share link

No response

Operating System

macOS and Linux

Terminal

No response