Implement WebSocketClient for receiving real-time plan update events
from a CleverAgents server via WebSocket connection:
- Plan update subscription with reconnect/exponential backoff
- Event schema for plan status, progress, and log stream updates
- Heartbeat/ping handling that resets reconnect counter
- Resume from last event ID on reconnect
- Event version negotiation
- Event de-duplication by event_id via LRU-bounded EventDeduplicator
- Ordered delivery guarantees with thread-safe callback dispatch
- Configurable reconnect backoff parameters (base, max, max_reconnects)
- ConnectionState model tracking lifecycle state
- websockets library added to pyproject.toml dependencies
- Behave scenarios (16 scenarios, 62 steps)
- Robot Framework smoke tests
- ASV benchmark for message handling baseline
- Reference documentation at docs/reference/server_websocket.md
ISSUES CLOSED: #337
