refactor: rename memory_entity→knowledge_node, knowledge graph edge→knowledge_edge

Resolves table name collision between:
- memory_edge (provenance DAG: child_sha/parent_sha) — KEPT
- knowledge_edge (knowledge graph: source_id/target_id) — NEW NAME

Changes:
- memory_entity → knowledge_node (all .rs + migrations 003-009)
- knowledge graph memory_edge → knowledge_edge
- memory_entity_version → knowledge_node_version
- memory_edge_version → knowledge_edge_version
- Added knowledge_node + knowledge_edge to init_schema()
- Converted versioning.rs from sqlx::query_as! to runtime queries
  (avoids stale sqlx offline cache dependency)
- Fixed UUID cast: $1::UUID for String→UUID column binds
- Fixed column names: source_entity_id→source_id, target_entity_id→target_id

Production DB: knowledge_node + knowledge_edge tables created,
memory_entity VIEW points to knowledge_node for backward compat.

crates/mem-ingest/src/grm_retriever.rs

@@ -204,7 +204,7 @@ impl GraphContextRetriever for PostgresGrmRetriever {
         debug!("PostgresGrmRetriever: get_entity_context({})", entity_name);
 
         // TODO (Phase 2.5): Implement actual database query
-        // SELECT id, name, summary FROM memory_entity
+        // SELECT id, name, summary FROM knowledge_node
         // WHERE name_embedding <-> query_embedding < (1 - threshold)
         // LIMIT max_entity_context_size
 
@@ -263,7 +263,7 @@ impl GraphContextRetriever for PostgresGrmRetriever {
         debug!("PostgresGrmRetriever: get_fact_context({})", fact_text);
 
         // TODO (Phase 2.5): Implement actual database query
-        // SELECT COUNT(*) FROM memory_edge
+        // SELECT COUNT(*) FROM knowledge_edge
         // WHERE source_id = ? AND target_id = ?
         //   AND fact_embedding <-> query_embedding < (1 - similarity_threshold)
         // AND (t_invalid IS NULL OR t_invalid > NOW())