feat(auth): centralize token store management and enhance persistence

- Introduced `RegisterTokenStore` and `GetTokenStore` to centralize token store access.
- Replaced direct file operations with a unified token persistence API.
- Updated all components to use the shared token store for consistent behavior.
- Improved logging for token save operations to include file paths.

internal/cmd/auth_manager.go

@@ -11,7 +11,7 @@ import (
 // Returns:
 //   - *sdkAuth.Manager: A configured authentication manager instance
 func newAuthManager() *sdkAuth.Manager {
-	store := sdkAuth.NewFileTokenStore()
+	store := sdkAuth.GetTokenStore()
 	manager := sdkAuth.NewManager(store,
 		sdkAuth.NewGeminiAuthenticator(),
 		sdkAuth.NewCodexAuthenticator(),