Previously, clusterByDomain() and clusterByTime() methods contained
stub implementations that always returned empty arrays. This caused
empty results when attempting domain-based or temporal clustering.
Changes:
- Implement _getItemsByField() to query brain storage
- Implement _getItemsByTimeWindow() to filter by time windows
- Fix _groupByDomain() to check root, metadata, and data fields
- Implement _findCrossDomainMembers() for cross-domain analysis
- Implement _findCrossDomainClusters() to merge similar clusters
- Add comprehensive tests for domain and time clustering
- Update documentation structure to include VFS guides
The methods now properly query the brain's storage, filter results,
and return functional clustering data.
