fix: resolve CI lint failures and clean up codebase - #144

GitHub Actions step uses a mutable tag or branch reference. Tags and branch names can be silently repointed by the action owner, enabling supply-chain attacks — as seen in the trivy-action and kics-github-action compromises. Pin the reference to a full 40-character commit SHA instead, e.g. uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608.