feat: complete Go backend control-plane parity

Bridge Java-to-Go runtime behavior by enforcing forward ownership checks, wiring node command dispatch/diagnostics, and adding contract coverage so migrated APIs can run with production semantics.

go-backend/internal/store/sqlite/repository.go

@@ -25,6 +25,13 @@ type Repository struct {
 	db *sql.DB
 }
 
+func (r *Repository) DB() *sql.DB {
+	if r == nil {
+		return nil
+	}
+	return r.db
+}
+
 type User struct {
 	ID            int64
 	User          string