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/rollout/mod.rs

@@ -0,0 +1,12 @@
+//! Rollout module: persistence and discovery of session rollout files.
+
+pub(crate) const SESSIONS_SUBDIR: &str = "sessions";
+
+pub mod list;
+pub mod recorder;
+
+pub use recorder::RolloutRecorder;
+pub use recorder::SessionStateSnapshot;
+
+#[cfg(test)]
+pub mod tests;