feat(memory): FTS5 index for graph entities #1232
Description
Summary
Follow-up to graph memory epic (#1222). When entity count exceeds ~10K, the current LIKE '%query%' fuzzy search becomes slow. Add FTS5 virtual table for graph_entities (name + summary).
Internal implementation change in GraphStore::find_entities_fuzzy — no public API change needed.
Depends on: #1224 (schema & types)