This commit updates the `README_MAS.md` to include important notes on keychain trust settings and the installation of Apple intermediate certificates, addressing common codesigning issues. Additionally, the `build_mas_pkg.sh` script is enhanced with functions to prepare the login keychain for signing, diagnose chain failures, and provide clear error messages for authorization issues during the build process. These improvements aim to streamline the macOS build experience and assist developers in resolving keychain-related errors effectively.