feat: dynamic model fetching for GitHub Copilot

- Add ListModels/ListModelsWithGitHubToken to CopilotAuth for querying
  the /models endpoint at api.githubcopilot.com
- Add FetchGitHubCopilotModels in executor with static fallback on failure
- Update service.go to use dynamic fetching (15s timeout) instead of
  hardcoded GetGitHubCopilotModels()
- Add GitHubCopilotAliasesFromModels for auto-generating dot-to-hyphen
  model aliases from dynamic model lists

sdk/cliproxy/service.go

@@ -866,7 +866,9 @@ func (s *Service) registerModelsForAuth(a *coreauth.Auth) {
 		models = registry.GetKimiModels()
     models = applyExcludedModels(models, excluded)
 	case "github-copilot":
-		models = registry.GetGitHubCopilotModels()
+		ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+		models = executor.FetchGitHubCopilotModels(ctx, a, s.cfg)
+		cancel()
 		models = applyExcludedModels(models, excluded)
 	case "kiro":
 		models = s.fetchKiroModels(a)