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.

assets/item/air.json

@@ -0,0 +1,5 @@
+{
+    "id": 0,
+    "name": "air",
+    "description": ""
+}