fix: correct GitHub Actions versions in CI and publish workflows (#10)

actions/checkout@v6 and actions/setup-python@v6 do not exist in the
GitHub Marketplace — the latest stable versions are v4 and v5 respectively.
Using nonexistent tags causes all workflow runs to fail immediately with
resolution errors.

Fixed both ci.yml and publish.yml to use the correct stable versions.

Co-authored-by: DevForge Engineer <engineer@devforge.dev>

Author: Coding-Dev-Tools