Replace actions/checkout and setup-python with internal git clone and system python3. Drops CI matrix to a single job on the self-hosted runner.
ubuntu-latest never matched the self-hosted runner; Gitea also rejects GITEA_-prefixed secret names. Wire publish workflow to PACKAGE_USER/TOKEN.
Add make package-check/publish-gitea, tag-triggered Gitea Actions workflow, PACKAGE_RELEASE.md, and update README/GETTING_STARTED install instructions for the Coulomb registry (v1.1.0+).