Improve clarity around how you can authenticate

### Describe the feature or problem you'd like to solve

Today, I ran `/logout` and closed copilot. Then ran copilot and expected I would need to authenticate, but I did not because it automagically used by token from gh cli.

### Proposed solution

When checking for authentication say something like:

Checking for a GitHub Token in your environment, copilot /login, or gh cli login.

Using X.

When using /logout,
- If using environment or gh cli login, inform the user that they need to clear those tokens.

Optional: Allow configuration to not use the gh cli token or ask permission to use it.

### Example prompts or workflows

- Login into gh cli.
- Run Copilot
- Run /Logout
- Close Copilot
- Run Copilot. See that you still login magically when maybe you expected not to.

### Additional context

_No response_