core(rollout): extract rollout module, add listing API, and return file heads (#1634)

- Move rollout persistence and listing into a dedicated module:
rollout/{recorder,list}.
- Expose lightweight conversation listing that returns file paths plus
the first 5 JSONL records for preview.

codex-rs/core/src/lib.rs

@@ -59,6 +59,7 @@ pub mod terminal;
 mod tool_apply_patch;
 pub mod turn_diff_tracker;
 pub mod user_agent;
+pub use rollout::list::ConversationsPage;
 mod user_notification;
 pub mod util;
 pub use apply_patch::CODEX_APPLY_PATCH_ARG1;