Adds eng/clean-materialized-plugins.mjs which removes agents/, commands/,
and skills/ subdirectories from plugin folders. These are generated by CI
via eng/materialize-plugins.mjs during publish and should not be tracked on
the staged branch.
Runnable via: npm run plugin:clean
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
