Engine-parity host polish: index page, pretty JSON, engine error shape - #19

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/

changes touched tracked files but added no memory/sessions/ log — write or enrich a session log per memory/PROTOCOL.md "Close every session".