fix(agent-harness): structured policy-block messages — rework of #4390 onto post-#4399 - #4443

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: dorny/paths-filter@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/