feat(item): add item manager with JSON asset loading

Implement ItemManager to load and query item definitions from assets/item JSON files. Add ItemData struct, item asset metadata, CMake source registration, and initialize the manager during app startup. Also add AGENTS.md repository guidelines.

src/CMakeLists.txt

@@ -104,4 +104,5 @@ target_sources(${PROJECT_NAME}
         gameplay/client_entity_manager.cpp
         gameplay/systems/wander_ai_system.cpp
         tools/json_utils.cpp
+        gameplay/item_manager.cpp
 )