feat: tag and publish a Gitea release on every push to master

A plugin installed as a bare git+https URL is pinned to a commit, so with no
tags there is nothing for `dsh plugin update` to move to. The workflow reads
the commit subjects since the last tag, bumps package.json, prepends
CHANGELOG.md, tags vX.Y.Z, and publishes the Gitea release with those subjects
as the body. The decision logic lives in .gitea/scripts/release.sh so it runs
and is tested without a runner (`npm run test:release`).

CHANGELOG.md

@@ -0,0 +1,3 @@
+# Changelog
+
+Every release, newest first, generated from the commits in each release.