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>

cmd/exo/main.go

@@ -29,6 +29,8 @@ Commands:
   cat add <category> [...]    Create categories
   cat list                    List all categories
   search tag <tag>            Search artifacts by tag
+  node create <name>          Create a knowledge graph node
+  node get <id>               Get a node with its cells
   version                     Print version
 
 Environment:
@@ -53,13 +55,15 @@ func main() {
 		runCat(os.Args[2:])
 	case "search":
 		runSearch(os.Args[2:])
+	case "node":
+		runNode(os.Args[2:])
 	default:
 		fmt.Fprintf(os.Stderr, "unknown command: %s\n", os.Args[1])
 		usage()
 	}
 }
 
-func dial() pb.ArtifactServiceClient {
+func dialConn() *grpc.ClientConn {
 	addr := os.Getenv("EXO_ADDR")
 	if addr == "" {
 		addr = "localhost:9090"
@@ -70,8 +74,15 @@ func dial() pb.ArtifactServiceClient {
 	if err != nil {
 		log.Fatalf("exo: failed to connect to %s: %v", addr, err)
 	}
-	// Connection will be closed when the process exits.
-	return pb.NewArtifactServiceClient(conn)
+	return conn
+}
+
+func dial() pb.ArtifactServiceClient {
+	return pb.NewArtifactServiceClient(dialConn())
+}
+
+func dialKG() pb.KnowledgeGraphServiceClient {
+	return pb.NewKnowledgeGraphServiceClient(dialConn())
 }
 
 func runImport(args []string) {
@@ -203,3 +214,62 @@ func runSearch(args []string) {
 		os.Exit(1)
 	}
 }
+
+func runNode(args []string) {
+	if len(args) == 0 {
+		fmt.Fprintln(os.Stderr, "usage: exo node <create|get> [...]")
+		os.Exit(1)
+	}
+
+	client := dialKG()
+	ctx := context.Background()
+
+	switch args[0] {
+	case "create":
+		if len(args) < 2 {
+			fmt.Fprintln(os.Stderr, "usage: exo node create <name>")
+			os.Exit(1)
+		}
+		resp, err := client.CreateNode(ctx, &pb.CreateNodeRequest{
+			Name: args[1],
+			Type: "note",
+		})
+		if err != nil {
+			log.Fatalf("exo: failed to create node: %v", err)
+		}
+		fmt.Printf("created node: %s\n", resp.Id)
+
+	case "get":
+		if len(args) < 2 {
+			fmt.Fprintln(os.Stderr, "usage: exo node get <id>")
+			os.Exit(1)
+		}
+		resp, err := client.GetNode(ctx, &pb.GetNodeRequest{Id: args[1]})
+		if err != nil {
+			log.Fatalf("exo: failed to get node: %v", err)
+		}
+		n := resp.Node
+		fmt.Printf("ID:       %s\n", n.Id)
+		fmt.Printf("Name:     %s\n", n.Name)
+		fmt.Printf("Type:     %s\n", n.Type)
+		fmt.Printf("Parent:   %s\n", n.ParentId)
+		fmt.Printf("Created:  %s\n", n.Created)
+		fmt.Printf("Modified: %s\n", n.Modified)
+		if len(n.Children) > 0 {
+			fmt.Printf("Children: %v\n", n.Children)
+		}
+		if len(n.Tags) > 0 {
+			fmt.Printf("Tags:     %v\n", n.Tags)
+		}
+		if len(resp.Cells) > 0 {
+			fmt.Printf("Cells:    %d\n", len(resp.Cells))
+			for _, c := range resp.Cells {
+				fmt.Printf("  [%d] %s (%s): %s\n", c.Ordinal, c.Id, c.Type, string(c.Contents))
+			}
+		}
+
+	default:
+		fmt.Fprintf(os.Stderr, "unknown node subcommand: %s\n", args[0])
+		os.Exit(1)
+	}
+}

cmd/exod/main.go

@@ -71,6 +71,7 @@ func main() {
 
 	grpcServer := grpc.NewServer()
 	pb.RegisterArtifactServiceServer(grpcServer, server.NewArtifactServer(database, blobStore))
+	pb.RegisterKnowledgeGraphServiceServer(grpcServer, server.NewKGServer(database))
 
 	// Graceful shutdown on SIGINT/SIGTERM.
 	sigCh := make(chan os.Signal, 1)