## What changed
- Allow `thread/start` to select experimental `historyMode: "paginated"` when the thread store supports turn and item pagination.
- Resume paginated threads with bounded model context and require `excludeTurns: true` instead of loading full history.
- Return `turnsBackwardsCursor` and `itemsBackwardsCursor` from `thread/resume` so clients can hydrate durable history through `thread/turns/list` and `thread/items/list` while newer records arrive through live notifications.
- Reject operations that still require full history, including `initialTurnsPage`, `thread/read` with `includeTurns: true`, `thread/rollback`, and detached review.
- Make item cursors independent of the optional `turnId` filter.
## Testing
- Cover paginated thread start, cold and live resume, backwards cursor hydration, metadata reads, unsupported operations, and stores without pagination support.
GitOrigin-RevId: c42f9b268e29d049843330f474fa3ecde12f5b8a
Owen Lin
da61f7d8e1