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.

internal/cmd/github_copilot_login.go

@@ -0,0 +1,44 @@
+package cmd
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	sdkAuth "github.com/router-for-me/CLIProxyAPI/v6/sdk/auth"
+	log "github.com/sirupsen/logrus"
+)
+
+// DoGitHubCopilotLogin triggers the OAuth device flow for GitHub Copilot and saves tokens.
+// It initiates the device flow authentication, displays the user code for the user to enter
+// at GitHub's verification URL, and waits for authorization before saving the tokens.
+//
+// Parameters:
+//   - cfg: The application configuration containing proxy and auth directory settings
+//   - options: Login options including browser behavior settings
+func DoGitHubCopilotLogin(cfg *config.Config, options *LoginOptions) {
+	if options == nil {
+		options = &LoginOptions{}
+	}
+
+	manager := newAuthManager()
+	authOpts := &sdkAuth.LoginOptions{
+		NoBrowser: options.NoBrowser,
+		Metadata:  map[string]string{},
+		Prompt:    options.Prompt,
+	}
+
+	record, savedPath, err := manager.Login(context.Background(), "github-copilot", cfg, authOpts)
+	if err != nil {
+		log.Errorf("GitHub Copilot authentication failed: %v", err)
+		return
+	}
+
+	if savedPath != "" {
+		fmt.Printf("Authentication saved to %s\n", savedPath)
+	}
+	if record != nil && record.Label != "" {
+		fmt.Printf("Authenticated as %s\n", record.Label)
+	}
+	fmt.Println("GitHub Copilot authentication successful!")
+}