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/list.rs

@@ -1082,6 +1082,9 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
             RolloutItem::Compacted(_) => {
                 // Not included in `head`; skip.
             }
+            RolloutItem::SessionState(_) => {
+                // Not included in `head`; skip.
+            }
             RolloutItem::EventMsg(ev) => {
                 if let EventMsg::UserMessage(user) = ev {
                     summary.saw_user_event = true;
@@ -1134,6 +1137,7 @@ pub async fn read_head_for_summary(path: &Path) -> io::Result<Vec<serde_json::Va
                     }
                 }
                 RolloutItem::Compacted(_)
+                | RolloutItem::SessionState(_)
                 | RolloutItem::TurnContext(_)
                 | RolloutItem::EventMsg(_) => {}
             }