- Add Unwrap() to AuthenticationError for proper error chain handling with errors.Is/As
- Extract hardcoded header values to constants for maintainability
- Replace verbose status code checks with isHTTPSuccess() helper
- Remove unused ExtractBearerToken() and BuildModelsURL() functions
- Make buildChatCompletionURL() private (only used internally)
- Remove unused 'strings' import
Add complete GitHub Copilot support including:
- Device flow OAuth authentication via GitHub's official client ID
- Token management with automatic caching (25 min TTL)
- OpenAI-compatible API executor for api.githubcopilot.com
- 16 model definitions (GPT-5 variants, Claude variants, Gemini, Grok, Raptor)
- CLI login command via -github-copilot-login flag
- SDK authenticator and refresh registry integration
Enables users to authenticate with their GitHub Copilot subscription and
use it as a backend provider alongside existing providers.