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/auth_manager.go

@@ -6,7 +6,7 @@ import (
 
 // newAuthManager creates a new authentication manager instance with all supported
 // authenticators and a file-based token store. It initializes authenticators for
-// Gemini, Codex, Claude, and Qwen providers.
+// Gemini, Codex, Claude, Qwen, IFlow, Antigravity, and GitHub Copilot providers.
 //
 // Returns:
 //   - *sdkAuth.Manager: A configured authentication manager instance
@@ -19,6 +19,7 @@ func newAuthManager() *sdkAuth.Manager {
 		sdkAuth.NewQwenAuthenticator(),
 		sdkAuth.NewIFlowAuthenticator(),
 		sdkAuth.NewAntigravityAuthenticator(),
+		sdkAuth.NewGitHubCopilotAuthenticator(),
 	)
 	return manager
 }