Add stylelint to CI workflow

Complete the SCSS linting migration by adding stylelint to the CI pipeline.
This step was originally attempted but reverted due to permission constraints.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: 2 people

.github/workflows/lint-js-and-ruby.yml

@@ -113,6 +113,8 @@ jobs:
         run: yarn run eslint --report-unused-disable-directives
       - name: Check formatting
         run: yarn start format.listDifferent
+      - name: Lint SCSS with stylelint
+        run: yarn run lint:scss
       - name: Type-check TypeScript
         run: yarn run type-check
       - name: Pack for attw and publint