fix(apiproxy): derive queued steering on the client, drop it from the wire

The session/queued frame no longer carries steering — AgentMessage no longer
has the field. The client derives it from the same ordered turn boundaries the
host saw (a frame arriving while a turn is open joined the steering FIFO).

packages/client/ui-subagent/tests/browser-plugin.spec.ts

@@ -22,7 +22,7 @@ function summary(partial: Partial<SessionSummary> & { id: SessionId }): SessionS
     running: false,
     updatedAt: 0,
     ...partial,
-  }
+  } as SessionSummary
 }
 
 const sid = (id: string) => id as SessionId