`copilot` should not use whatever valid token it can find if I set `GITHUB_TOKEN`

[williammartin]

Describe the feature or problem you'd like to solve

If I try to auth via env var, I expect copilot to use it even if it is invalid, and not to fallback to another source like the keyring or gh. This can result in highly confusing behaviour when scopes or accessible resources are different.

Proposed solution

copilot should error, indicating that the token in was invalid.

Example prompts or workflows

➜  runc git:(main) GITHUB_TOKEN=foo copilot -p "hello"
● Hello! I'm GitHub Copilot CLI, ready to help you with tasks in your runc repository. What would
   you like to do?


Total usage est:       1 Premium request
Total duration (API):  3.4s
Total duration (wall): 5.2s
Total code changes:    0 lines added, 0 lines removed
Usage by model:
    claude-sonnet-4.5    5.9k input, 33 output, 2.1k cache read, 3.8k cache write (Est. 1 Premium request)

Additional context

No response