Add weekly check for new semble releases

[navedmerchant]

Adds a scheduled GitHub Actions workflow that:

• Checks PyPI weekly (Mondays 9am UTC) for new semble versions
• Compares against existing git tags to avoid duplicate builds
• When a new version is found, updates pyproject.toml, refreshes the lockfile, and pushes a version tag
• The new tag automatically triggers the existing build workflow to produce executables

Can also be triggered manually via workflow_dispatch.