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/stone.json

@@ -0,0 +1,6 @@
+{
+    "id": 3,
+    "name": "stone",
+    "description": "",
+    "texture": "texture/item/block/stone.png"
+}