slice 1.0.10: WebSocket client service for /ws/demo

**Phase 1.0 demo slice · Difficulty:** D2

## Scope
Injectable Angular service. Connects to `/ws/demo`, sends `{prompt}`, parses incoming events, pushes them into a `WritableSignal<Event[]>`.

No reconnect logic — refresh on disconnect is fine for the slice.

## Acceptance criteria
- [ ] Typed events match the agent-core event union.
- [ ] Service spec covers connect / send / receive / close.
