The runner now lives on Gitea; move the workflow to .gitea/workflows/ and
delete the stale .github copy (Gitea ignores .github/, so it's drift bait).
Use the native gitea.ref context for the tag gate; keep actions pinned to
their GitHub sources (the runner has internet + caches them). Drop the
upload-artifact coverage step in favour of the self-hosted webserver plan
tracked in the backlog. Add an empty-secret gate as publish's first step so
a tag push without NPM_TOKEN fails loudly instead of silently no-oppping.
