Auto cleanup as data grows

Is there an easy way to clean up docs in a groundDB to prevent it from growing? Should we simply listen to `isDone` event and call `keep` when it goes over a number of docs?

Review the GroundDB APIs and usage around the `isDone` event and `keep` method first. Clarify whether cleanup should be triggered automatically at a document-count threshold, and define the retention behavior and completion criteria before implementation.

Written by the indexing model from the issue text.