feat(mesh): implement Phase 6 Rich Health Metrics ...

- Enhanced agent-node to collect per-core CPU, frequency, available RAM, and load avg.
- Updated gRPC server to ingest and propagate rich health metrics via heartbeats.
- Updated Pydantic schemas to expose rich metrics in REST and WebSocket snapshots.
- Fixed UI tooltips in NodesPage.js to pop downwards and show detailed statistics.
- Fixed RegistrationResponse field mismatch (message -> error_message).
- Updated feature documentation for Agent Node Mesh.

Fix Nodes UI WebSocket and metrics, add feature documentation