feat: agent session management — metrics, channels, dispose (P2-006) (#78)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/chat/chat.gateway.ts

@@ -50,6 +50,7 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     const session = this.clientSessions.get(client.id);
     if (session) {
       session.cleanup();
+      this.agentService.removeChannel(session.conversationId, `websocket:${client.id}`);
       this.clientSessions.delete(client.id);
     }
   }
@@ -97,6 +98,9 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
 
     this.clientSessions.set(client.id, { conversationId, cleanup });
 
+    // Track channel connection
+    this.agentService.addChannel(conversationId, `websocket:${client.id}`);
+
     // Send acknowledgment
     client.emit('message:ack', { conversationId, messageId: uuid() });