Improve workflow for minimal auth scope vs gh auth login (via /login)

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

There is no easy way to authenticate Copilot CLI with minimal, scoped permissions in isolated environments. The `gh auth login` device code flow only adds to existing scopes and does not support per-repo granularity. A `/login` command or similar mechanism that supports scoped auth would reduce the need for workarounds.

This is especially important when running Copilot CLI in isolated environments with YOLO mode.

### Proposed solution

**Current workaround:** Create a Fine-grained Personal Access Token with `User: Copilot` and `Repository: Contents (read/write)` permissions, then authenticate with `gh auth login --with-token` or pass in `GH_TOKEN` env var.

### Example prompts or workflows

_No response_

### Additional context

_No response_