feat: support multiple subscribers for the same live session

[smallnest]

A single IMClaw session should be observable by multiple clients at the same time, such as CLI, web UI, and monitoring components.

Goals

• Allow more than one client to subscribe to the same live session stream
• Ensure event ordering remains stable for all subscribers
• Avoid coupling session lifetime to a single WebSocket connection

Deliverables

• Session stream fan-out design
• Subscription management in gateway
• Backpressure and disconnect handling
• Tests for multi-subscriber behavior

Acceptance criteria

• Multiple clients can observe the same session concurrently
• One subscriber disconnecting does not terminate the session
• Message ordering remains consistent