Persist agent messages as response items (#29829)

## Why

Inter-agent messages are recorded in live history as
`ResponseItem::AgentMessage`, but rollouts stored
`InterAgentCommunication` and rebuilt the response item during resume.
This made the rollout differ from the actual Responses history.

## What changed

- store the prepared `agent_message` response item directly
- keep `trigger_turn` in a small local metadata record for fork
truncation
- keep reading older `inter_agent_communication` rollout items

codex-rs/memories/write/src/phase1.rs

@@ -412,6 +412,7 @@ mod job {
                     Some(communication.to_model_input_item())
                 }
                 RolloutItem::SessionMeta(_)
+                | RolloutItem::InterAgentCommunicationMetadata { .. }
                 | RolloutItem::Compacted(_)
                 | RolloutItem::TurnContext(_)
                 | RolloutItem::EventMsg(_) => None,
@@ -790,6 +791,27 @@ mod tests {
         assert_eq!(parsed, expected);
     }
 
+    #[test]
+    fn serializes_agent_message_response_items_for_memory() {
+        let communication = InterAgentCommunication::new(
+            AgentPath::root(),
+            AgentPath::root().join("worker").expect("agent path"),
+            Vec::new(),
+            "delegated task".to_string(),
+            /*trigger_turn*/ true,
+        );
+        let response_item = communication.to_model_input_item();
+
+        let serialized = job::serialize_filtered_rollout_response_items(&[
+            RolloutItem::InterAgentCommunicationMetadata { trigger_turn: true },
+            RolloutItem::ResponseItem(response_item.clone()),
+        ])
+        .expect("serialize");
+        let parsed: Vec<ResponseItem> = serde_json::from_str(&serialized).expect("parse");
+
+        assert_eq!(parsed, vec![response_item]);
+    }
+
     #[test]
     fn count_outcomes_sums_token_usage_across_all_jobs() {
         let counts = aggregate_stats(vec![