Add persisted sections for organizing threads (#35722)

## What changed

- Replace the `isPinned` thread metadata and filters with an optional persisted
  `section` and `sectionId`.
- Add the paginated `threadSection/list` app-server method so clients can
  discover sections even when they contain no threads.
- Seed a stable `Pinned` section, validate section assignments, and support
  filtering for a specific section or for unsectioned threads.

## Testing

- Cover section protocol serialization, listing and pagination, metadata
  updates, filtering, persistence, migration compatibility, and operation
  without SQLite state.

GitOrigin-RevId: 7972b5471d29317b9387bfd90aa9f573f691ad4c

codex-rs/thread-store/src/local/archive_thread.rs

@@ -241,7 +241,7 @@ mod tests {
                 allowed_sources: Vec::new(),
                 model_providers: None,
                 cwd_filters: None,
-                is_pinned: None,
+                section: None,
                 archived: true,
                 search_term: None,
                 relation_filter: None,