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.

cmd/server/main.go

@@ -62,6 +62,7 @@ func main() {
 	var iflowCookie bool
 	var noBrowser bool
 	var antigravityLogin bool
+	var githubCopilotLogin bool
 	var projectID string
 	var vertexImport string
 	var configPath string
@@ -76,6 +77,7 @@ func main() {
 	flag.BoolVar(&iflowCookie, "iflow-cookie", false, "Login to iFlow using Cookie")
 	flag.BoolVar(&noBrowser, "no-browser", false, "Don't open browser automatically for OAuth")
 	flag.BoolVar(&antigravityLogin, "antigravity-login", false, "Login to Antigravity using OAuth")
+	flag.BoolVar(&githubCopilotLogin, "github-copilot-login", false, "Login to GitHub Copilot using device flow")
 	flag.StringVar(&projectID, "project_id", "", "Project ID (Gemini only, not required)")
 	flag.StringVar(&configPath, "config", DefaultConfigPath, "Configure File Path")
 	flag.StringVar(&vertexImport, "vertex-import", "", "Import Vertex service account key JSON file")
@@ -436,6 +438,9 @@ func main() {
 	} else if antigravityLogin {
 		// Handle Antigravity login
 		cmd.DoAntigravityLogin(cfg, options)
+	} else if githubCopilotLogin {
+		// Handle GitHub Copilot login
+		cmd.DoGitHubCopilotLogin(cfg, options)
 	} else if codexLogin {
 		// Handle Codex login
 		cmd.DoCodexLogin(cfg, options)