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/thread-store/src/thread_metadata_sync.rs

@@ -285,6 +285,7 @@ impl ThreadMetadataSync {
                 | RolloutItem::EventMsg(_)
                 | RolloutItem::ResponseItem(_)
                 | RolloutItem::InterAgentCommunication(_)
+                | RolloutItem::InterAgentCommunicationMetadata { .. }
                 | RolloutItem::Compacted(_) => {}
             }
         }