v2.3.1

Fixed

• release.yml now opens the GitHub Release for the tag it publishes. It only
  pushed to npm, so docs/package-guidelines.md shipped a rule — every tag gets
  a Release — that this repository was itself keeping by hand, and had already
  missed once on v2.0.0. The notes are the tag's own CHANGELOG.md section, so
  the two cannot drift, and a tag whose version has no entry now fails the
  release instead of shipping undocumented.