Update AgentChannel to only log (no state management)

Update AgentChannel to only log connections/disconnections. Status is managed by agents via MCP tools, not WebSocket state. ## Changes Required **File:** `app/channels/agent_channel.rb` ### subscribed - Log connection: `[AgentChannel] #{agent_type} connected` - Stream from `agent_#{agent_type}` - DO NOT update agent status (WebSocket is transient) ### unsubscribed - Log disconnection: `[AgentChannel] #{agent_type} disconnected` - DO NOT update agent status to offline (we don't have offline status) ## Streams - `agent_worker` - Workers subscribe - `agent_reviewer` - Reviewers subscribe - `agent_orchestrator` - Orchestrator subscribes (receives pings) ## Parent Epic #62 - Autonomous Agent Coordination via WebSocket