feat(auth): add GitHub Copilot authentication and API integration

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.

sdk/cliproxy/service.go

@@ -351,6 +351,8 @@ func (s *Service) ensureExecutorsForAuth(a *coreauth.Auth) {
 		s.coreManager.RegisterExecutor(executor.NewQwenExecutor(s.cfg))
 	case "iflow":
 		s.coreManager.RegisterExecutor(executor.NewIFlowExecutor(s.cfg))
+	case "github-copilot":
+		s.coreManager.RegisterExecutor(executor.NewGitHubCopilotExecutor(s.cfg))
 	default:
 		providerKey := strings.ToLower(strings.TrimSpace(a.Provider))
 		if providerKey == "" {
@@ -662,6 +664,8 @@ func (s *Service) registerModelsForAuth(a *coreauth.Auth) {
 		models = registry.GetQwenModels()
 	case "iflow":
 		models = registry.GetIFlowModels()
+	case "github-copilot":
+		models = registry.GetGitHubCopilotModels()
 	default:
 		// Handle OpenAI-compatibility providers by name using config
 		if s.cfg != nil {