Summary

Implements Phase 5 of the graph-based knowledge memory epic (#1222): community detection, graph eviction, and community refresh metrics.

• Community detection via label propagation (petgraph::UnGraph): groups entities into clusters (max 50 iterations, tie-break by smallest label, min 2 entities), generates LLM summaries, persists to graph_communities
• Incremental community assignment: new entities placed in nearest community by neighbor majority vote
• Graph eviction: deletes expired edges older than expired_edge_retention_days, orphan entities with no active edges, enforces optional max_entities cap — runs during community refresh cycle
• Atomic extraction counter via graph_metadata SQLite table (INSERT ON CONFLICT DO UPDATE)
• Prompt injection protection: scrub_content() applied to entity names and facts before LLM prompt
• Community detection failure metric: graph_community_detection_failures in MetricsSnapshot
• New GraphConfig fields: expired_edge_retention_days (default 90), max_entities (default 0)
• petgraph = "0.8" added as optional workspace dependency, gated behind graph-memory feature

Test plan

• 4300 tests pass (cargo nextest run --workspace --features full --lib --bins)
• 14 new tests: label propagation (12-entity graph, 4 clusters), single-entity filter, all-isolated-nodes, incremental assignment, eviction (expired edges, orphan entities), extraction counter (same-instance and cross-instance persistence via tempfile SQLite)

Follow-up issues filed

• perf(graph): guard community detection against large entity counts #1265 perf: guard community detection against large entity counts (stream instead of full materialization)
• perf(graph): concurrent LLM community summaries with FuturesUnordered #1266 perf: concurrent LLM summaries with FuturesUnordered
• docs(graph): document synchronous LPA behavior in community detection #1267 docs: document synchronous LPA behavior
• perf(graph): add index on graph_edges.expired_at for eviction queries #1268 perf: index on graph_edges.expired_at
• refactor(graph): newtype EntityId guard for BFS dynamic SQL #1269 refactor: EntityId newtype for BFS dynamic SQL
• refactor(graph): atomic UPSERT for extraction_count increment #1270 refactor: atomic UPSERT for extraction_count

Closes #1228