Add health check endpoint

[sebastientaggart]

Summary

Add a GET /health endpoint that returns service status, useful for monitoring and container orchestration.

Response should include

• Service uptime
• Number of connected WebSocket clients
• Number of registered agents and their statuses
• Plugin load status
• State store health (file writable, entry count)

[sebastientaggart]

Resuming work. Implementing GET /health (unauthenticated) with uptime, websocket client count, agent statuses, plugin load status, and state store health.