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/auth/copilot/token.go

@@ -0,0 +1,93 @@
+// Package copilot provides authentication and token management functionality
+// for GitHub Copilot AI services. It handles OAuth2 device flow token storage,
+// serialization, and retrieval for maintaining authenticated sessions with the Copilot API.
+package copilot
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/misc"
+)
+
+// CopilotTokenStorage stores OAuth2 token information for GitHub Copilot API authentication.
+// It maintains compatibility with the existing auth system while adding Copilot-specific fields
+// for managing access tokens and user account information.
+type CopilotTokenStorage struct {
+	// AccessToken is the OAuth2 access token used for authenticating API requests.
+	AccessToken string `json:"access_token"`
+	// TokenType is the type of token, typically "bearer".
+	TokenType string `json:"token_type"`
+	// Scope is the OAuth2 scope granted to the token.
+	Scope string `json:"scope"`
+	// ExpiresAt is the timestamp when the access token expires (if provided).
+	ExpiresAt string `json:"expires_at,omitempty"`
+	// Username is the GitHub username associated with this token.
+	Username string `json:"username"`
+	// Type indicates the authentication provider type, always "github-copilot" for this storage.
+	Type string `json:"type"`
+}
+
+// CopilotTokenData holds the raw OAuth token response from GitHub.
+type CopilotTokenData struct {
+	// AccessToken is the OAuth2 access token.
+	AccessToken string `json:"access_token"`
+	// TokenType is the type of token, typically "bearer".
+	TokenType string `json:"token_type"`
+	// Scope is the OAuth2 scope granted to the token.
+	Scope string `json:"scope"`
+}
+
+// CopilotAuthBundle bundles authentication data for storage.
+type CopilotAuthBundle struct {
+	// TokenData contains the OAuth token information.
+	TokenData *CopilotTokenData
+	// Username is the GitHub username.
+	Username string
+}
+
+// DeviceCodeResponse represents GitHub's device code response.
+type DeviceCodeResponse struct {
+	// DeviceCode is the device verification code.
+	DeviceCode string `json:"device_code"`
+	// UserCode is the code the user must enter at the verification URI.
+	UserCode string `json:"user_code"`
+	// VerificationURI is the URL where the user should enter the code.
+	VerificationURI string `json:"verification_uri"`
+	// ExpiresIn is the number of seconds until the device code expires.
+	ExpiresIn int `json:"expires_in"`
+	// Interval is the minimum number of seconds to wait between polling requests.
+	Interval int `json:"interval"`
+}
+
+// SaveTokenToFile serializes the Copilot token storage to a JSON file.
+// This method creates the necessary directory structure and writes the token
+// data in JSON format to the specified file path for persistent storage.
+//
+// Parameters:
+//   - authFilePath: The full path where the token file should be saved
+//
+// Returns:
+//   - error: An error if the operation fails, nil otherwise
+func (ts *CopilotTokenStorage) SaveTokenToFile(authFilePath string) error {
+	misc.LogSavingCredentials(authFilePath)
+	ts.Type = "github-copilot"
+	if err := os.MkdirAll(filepath.Dir(authFilePath), 0700); err != nil {
+		return fmt.Errorf("failed to create directory: %v", err)
+	}
+
+	f, err := os.Create(authFilePath)
+	if err != nil {
+		return fmt.Errorf("failed to create token file: %w", err)
+	}
+	defer func() {
+		_ = f.Close()
+	}()
+
+	if err = json.NewEncoder(f).Encode(ts); err != nil {
+		return fmt.Errorf("failed to write token to file: %w", err)
+	}
+	return nil
+}