Adds remark-gfm/directive/parse/rehype, rehype-pretty-code, shiki,
unified, unist-util-visit; retires remark/remark-html. Under PnP there
are no real node_modules dirs — cache zips live under paths *containing*
/node_modules/, so jest's transformIgnorePatterns now only excludes the
css module mocks, keeping the ESM-only unified chain transformable.