Implement P0 repository safety

[dudarev]

Summary

• Add GitHub Actions CI with tox coverage for Python 3.12 and 3.13.
• Add installed-package smoke checks for CLI entrypoints and import ytt.
• Remove the stale top-level ytt package and document the required main branch protection policy.

Verification

• python3 -m pytest
• tox -e py313
• PYENV_VERSION=3.12.9 tox -e py312