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/state/src/runtime/threads.rs

@@ -1163,6 +1163,7 @@ pub(super) fn extract_memory_mode(items: &[RolloutItem]) -> Option<String> {
         RolloutItem::SessionMeta(meta_line) => meta_line.meta.memory_mode.clone(),
         RolloutItem::ResponseItem(_)
         | RolloutItem::InterAgentCommunication(_)
+        | RolloutItem::InterAgentCommunicationMetadata { .. }
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,