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/rollout/src/list.rs

@@ -1151,6 +1151,7 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
                     .created_at
                     .get_or_insert_with(|| rollout_line.timestamp.clone());
             }
+            RolloutItem::InterAgentCommunicationMetadata { .. } => {}
             RolloutItem::TurnContext(_) => {
                 // Not included in `head`; skip.
             }
@@ -1213,7 +1214,8 @@ pub async fn read_head_for_summary(path: &Path) -> io::Result<Vec<serde_json::Va
                         head.push(value);
                     }
                 }
-                RolloutItem::Compacted(_)
+                RolloutItem::InterAgentCommunicationMetadata { .. }
+                | RolloutItem::Compacted(_)
                 | RolloutItem::TurnContext(_)
                 | RolloutItem::EventMsg(_) => {}
             }