The existing Copilot provider passes a static Bearer token, but the
Copilot API requires short-lived session tokens obtained via GitHub's
OAuth device code flow, plus mandatory editor headers.
This replaces the stub with a dedicated CopilotProvider that:
- Runs the OAuth device code flow on first use (same client ID as VS Code)
- Exchanges the OAuth token for a Copilot API key via
api.github.com/copilot_internal/v2/token
- Sends required Editor-Version/Editor-Plugin-Version headers
- Caches tokens to disk (~/.config/zeroclaw/copilot/) with auto-refresh
- Uses Mutex to prevent concurrent refresh races / duplicate device prompts
- Writes token files with 0600 permissions (owner-only)
- Respects GitHub's polling interval and code expiry from device flow
- Sanitizes error messages to prevent token leakage
- Uses async filesystem I/O (tokio::fs) throughout
- Optionally accepts a pre-supplied GitHub token via config api_key
Fixes: 403 'Access to this endpoint is forbidden'
Fixes: 400 'missing Editor-Version header for IDE auth'
