feat(demo): add a doctor command that says how to fix each problem - #22

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/