Add persisted manual ordering for thread sections (#36007)

## What changed

- Add `thread/section/move` to atomically move a thread into, within, or out of a section. Threads can be inserted before an existing member or appended, and moves within a section preserve `sectionEnteredAt`.
- Add `section_position` sorting to `thread/list`, with ascending order as its default, and expose `sectionEnteredAt` in thread responses.
- Persist section positions and entry times in SQLite, including migration of existing section members into recency order. Section membership is no longer updated through `thread/metadata/update`.

## Testing

- Cover section moves, reordering, pagination, persistence across restarts and rollout reconciliation, concurrent updates, and rank renumbering.

GitOrigin-RevId: aec6d7ddedca5277029b5caf5c074975397e956c

codex-rs/exec/src/lib_tests.rs

@@ -336,6 +336,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         preview: String::new(),
         ephemeral: false,
         section: None,
+        section_entered_at: None,
         history_mode: Default::default(),
         model_provider: "openai".to_string(),
         created_at: 0,
@@ -804,6 +805,7 @@ fn sample_thread_start_response() -> ThreadStartResponse {
             preview: String::new(),
             ephemeral: false,
             section: None,
+            section_entered_at: None,
             history_mode: Default::default(),
             model_provider: "openai".to_string(),
             created_at: 0,