Persist and prewarm agent tasks per thread (#17978)

## Summary
- persist registered agent tasks in the session state update stream so
the thread can reuse them
- prewarm task registration once identity registration succeeds, while
keeping startup failures best-effort
- isolate the session-side task lifecycle into a dedicated module so
AgentIdentityManager and RegisteredAgentTask do not leak across as many
core layers

## Testing
- cargo test -p codex-core startup_agent_task_prewarm
- cargo test -p codex-core
cached_agent_task_for_current_identity_clears_stale_task
- cargo test -p codex-core record_initial_history_

codex-rs/rollout/src/policy.rs

@@ -16,9 +16,10 @@ pub fn is_persisted_response_item(item: &RolloutItem, mode: EventPersistenceMode
         RolloutItem::ResponseItem(item) => should_persist_response_item(item),
         RolloutItem::EventMsg(ev) => should_persist_event_msg(ev, mode),
         // Persist Codex executive markers so we can analyze flows (e.g., compaction, API turns).
-        RolloutItem::Compacted(_) | RolloutItem::TurnContext(_) | RolloutItem::SessionMeta(_) => {
-            true
-        }
+        RolloutItem::Compacted(_)
+        | RolloutItem::TurnContext(_)
+        | RolloutItem::SessionMeta(_)
+        | RolloutItem::SessionState(_) => true,
     }
 }