Runs one chunk of a sync — a single page of 25 to 50 CRM records — and returns a checkpoint describing the progress. Keep calling it with the returned checkpointId while hasMore is true. It writes the contact mappings and the phone-number index that GET /api/v1/crm/lookup/phone reads. A chunk that fails is still reported with HTTP 200 and success: false, so check the body rather than the status.