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/state/src/extract.rs

@@ -19,6 +19,7 @@ pub fn apply_rollout_item(
 ) {
     match item {
         RolloutItem::SessionMeta(meta_line) => apply_session_meta_from_item(metadata, meta_line),
+        RolloutItem::SessionState(_) => {}
         RolloutItem::TurnContext(turn_ctx) => apply_turn_context(metadata, turn_ctx),
         RolloutItem::EventMsg(event) => apply_event_msg(metadata, event),
         RolloutItem::ResponseItem(item) => apply_response_item(metadata, item),
@@ -36,9 +37,10 @@ pub fn rollout_item_affects_thread_metadata(item: &RolloutItem) -> bool {
         RolloutItem::EventMsg(
             EventMsg::TokenCount(_) | EventMsg::UserMessage(_) | EventMsg::ThreadNameUpdated(_),
         ) => true,
-        RolloutItem::EventMsg(_) | RolloutItem::ResponseItem(_) | RolloutItem::Compacted(_) => {
-            false
-        }
+        RolloutItem::SessionState(_)
+        | RolloutItem::EventMsg(_)
+        | RolloutItem::ResponseItem(_)
+        | RolloutItem::Compacted(_) => false,
     }
 }