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

Replicate the dsh-facet release stack: a Gitea Actions workflow
(.gitea/workflows/release.yml) + .gitea/scripts/release.sh read the commit
subjects since the last tag, bump package.json, prepend CHANGELOG.md, tag
vX.Y.Z and publish the release via API. Agreed install ref is
#semver:^0.1.0, and tags are what 'dsh plugin --profile web update' moves
to, so the workflow is what makes updates work. Add the release.test.sh
suite (43 checks), npm scripts (test:release / release:plan /
release:notes), AGENTS.md with the commit conventions, CHANGELOG.md seed,
and the README *Releases* rules.

CHANGELOG.md

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