fix(ws): broadcast cron job results to WebSocket clients in real-time

[nimitbhardwaj]

Summary

Fixes #1088 — Scheduled task results now appear in the web UI in real-time via WebSocket push, without requiring a page refresh.

Problem

When a cron job fires, the result was only delivered to configured channels (Telegram, etc.) via cron_deliver_response. WebSocket clients had no mechanism to receive these results — they had to refresh the page to see cron output.

Solution

Backend

• WS Connection Registry: Added global tracking of WebSocket connections per agent (ws.rs). Connections register on connect and deregister on disconnect.
• CronJobExecuted Event: New SystemEvent::CronJobExecuted variant emitted by cron_run_job after the agent loop completes.
• WS Broadcaster: start_ws_cron_broadcaster() subscribes to the kernel event bus and broadcasts to all active WebSocket connections for the relevant agent.
• Dual Delivery: Cron results still go to channels via cron_deliver_response (unchanged), and now also broadcast to WebSocket.

Frontend

• Added handler for type: "message" events from server — these display as user messages (left side) with [Scheduled: job_name] label.

Testing

1. Create a schedule (e.g., every 1 min: "Tell the user 'Hello World'")
2. Open web UI and connect to agent via WebSocket
3. Observe cron trigger message appears on left, response on right, in real-time
4. Multiple browser tabs all receive the broadcast

Breaking Changes

None.