Add Phase 4 knowledge graph: nodes, cells, facts, edges, gRPC service

Build the knowledge graph pillar with the kg package:
- Node: hierarchical notes with parent/children, C2 wiki-style naming,
  shared tag/category pool with artifacts
- Cell: content units (markdown, code, plain) with ordinal ordering
- Fact: EAV tuples with transaction timestamps and retraction support
- Edge: directed graph links (child, parent, related, artifact_link)

Includes schema migration (002_knowledge_graph.sql), protobuf definitions
(kg.proto), full gRPC KnowledgeGraphService implementation, CLI commands
(node create/get), and comprehensive test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

db/db_test.go

@@ -197,7 +197,7 @@ func TestSchemaVersion(t *testing.T) {
 	if err != nil {
 		t.Fatalf("getCurrentVersion failed: %v", err)
 	}
-	if version != 1 {
-		t.Fatalf("expected schema version 1, got %d", version)
+	if version < 1 {
+		t.Fatalf("expected schema version >= 1, got %d", version)
 	}
 }