Add GET /health endpoint

[sebastientaggart]

Adds an unauthenticated GET /health endpoint for monitoring and container orchestration.

Response includes:

• Service uptime and version
• WebSocket client count
• Agent count and per-agent statuses
• Plugin modules, action count, signal count
• State store entry count, persist path, and writable check

Closes #6

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

No issues found. Code looks correct and follows project conventions.

• Health endpoint correctly guards against uninitialized globals with a 503.
• No secrets, no auth bypass concerns beyond the intentional unauthenticated /health.
• is_writable and entry_count additions are minimal and well-scoped.
• Test covers the unauthenticated path and key response fields.