Publish the 13-criteria PR review checklist the maintainer runs on
every incoming PR so contributors (and their coding agents) can run
the same pass before opening, surfacing problems before the review
round trip. CONTRIBUTING.md gets a new 'Self-review against the
project review checklist' subsection under 'Submitting pull requests'
and a Further Reading entry. CHANGELOG [Unreleased] gets an Added
entry.
