fix: revert Node version to 18 in CI workflow

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite

.github/workflows/integrity-check.yml

@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version: '18'
       - run: yarn
       - run: yarn lint
       - run: yarn test:ci --coverage