feat(observability): propagate optional cost_usd on agent end

src/agent/agent.rs

@@ -557,6 +557,7 @@ pub async fn run(
     agent.observer.record_event(&ObserverEvent::AgentEnd {
         duration: start.elapsed(),
         tokens_used: None,
+        cost_usd: None,
     });
 
     Ok(())

src/agent/loop_.rs

@@ -1048,6 +1048,7 @@ pub async fn run(
     observer.record_event(&ObserverEvent::AgentEnd {
         duration,
         tokens_used: None,
+        cost_usd: None,
     });
 
     Ok(final_output)