Agent presence indicator: show online/offline dot on agent cards

[leocagli]

Context

The agent registry lists agents but there's no visual indicator of whether an agent is currently online (sending heartbeats). Adding a presence dot makes the registry actionable — users can tell at a glance which agents are active.

What to implement

Presence endpoint (already exists via health store)

GET /api/agents/:id/health returns status: 'healthy' | 'offline'. The registry page already fetches agent cards — augment each with a presence fetch.

UI change in components/agents-registry.tsx

Add a small dot overlay on each agent card avatar:

• Green dot: status === 'healthy' (last heartbeat < 30s ago)
• Red dot: status === 'offline'
• Grey dot: health endpoint returns 404 (never sent a heartbeat)

Use CSS absolute positioning over the avatar. Dots are 10px, border-white, rounded-full.

Bulk presence endpoint (perf)

Rather than N individual health fetches, add GET /api/agents/presence?ids=id1,id2,id3 returning a map of { agentId: status } in one call (max 50 IDs).

Acceptance criteria

• Agent card shows green dot when agent has recent heartbeat
• Agent card shows red dot when agent is offline
• Agent card shows grey dot when agent has never sent heartbeat
• GET /api/agents/presence?ids=... returns status for all requested agents
• Unit test for presence endpoint: mix of healthy, offline, unknown agents

🎁 Evidencia visual = reward extra

• Si tu PR incluye video demo o capturas de pantalla mostrando la funcionalidad pedida funcionando end-to-end, va a ser considerado para rewards de GrantFox en esta issue. No es obligatorio, pero suma mucho para la evaluación.