Add basic operational metrics

[sebastientaggart]

Summary

Add lightweight metrics tracking for operational visibility. No need for Prometheus/Grafana initially — expose via a GET /metrics endpoint or include in the health check response.

Metrics to track

• Connected WebSocket clients (current count)
• Events emitted per second / total
• Actions executed (count, success/failure)
• Signals received (count, by type)
• Agent status distribution (idle/running/awaiting/error)
• State store size (entry count, approximate memory)