theme.json regenerated, and the check that has fired five times

A generated file under version control, and the CI check that is the only thing keeping the generation meaningful.

- run: |
    node build/tokens.mjs
    git diff --exit-code theme.json assets/css/tokens.css 
      || { echo '::error::run node build/tokens.mjs'; exit 1; }

Five failures in four years, every one of them somebody editing theme.json directly because it is the file they found when looking for a colour. A generated artefact under version control needs this or the generation becomes advisory, and the failure without it is silent — the next build overwrites the edit.