feat: intelligent review & scan — wire symbol index, code graph, brain search + nomic embed (#455, #456, #457, #458) - #461

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/