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/protocol/src/protocol.rs

@@ -2599,6 +2599,7 @@ impl InitialHistory {
                 RolloutItem::SessionMeta(_)
                 | RolloutItem::ResponseItem(_)
                 | RolloutItem::InterAgentCommunication(_)
+                | RolloutItem::InterAgentCommunicationMetadata { .. }
                 | RolloutItem::Compacted(_)
                 | RolloutItem::EventMsg(_) => None,
             })
@@ -2933,6 +2934,7 @@ fn multi_agent_version_from_items(
             RolloutItem::SessionMeta(_)
             | RolloutItem::ResponseItem(_)
             | RolloutItem::InterAgentCommunication(_)
+            | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::Compacted(_)
             | RolloutItem::EventMsg(_) => None,
         })
@@ -3080,8 +3082,12 @@ impl<'de> Deserialize<'de> for SessionMetaLine {
 pub enum RolloutItem {
     SessionMeta(SessionMetaLine),
     ResponseItem(ResponseItem),
-    /// Durable delivery metadata reconstructed as a model-visible `agent_message`.
+    /// Legacy delivery item reconstructed as a model-visible `agent_message`.
     InterAgentCommunication(InterAgentCommunication),
+    /// Local delivery metadata that is not part of the Responses API item.
+    InterAgentCommunicationMetadata {
+        trigger_turn: bool,
+    },
     Compacted(CompactedItem),
     TurnContext(TurnContextItem),
     EventMsg(EventMsg),