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": ""
+}

assets/item/dirt.json

@@ -0,0 +1,6 @@
+{
+    "id": 2,
+    "name": "dirt",
+    "description": "",
+    "texture": "texture/item/block/dirt.png"
+}

assets/item/grass.json

@@ -0,0 +1,6 @@
+{
+    "id": 9,
+    "name": "grass",
+    "description": "",
+    "texture": "texture/item/block/grass.png"
+}

assets/item/grass_block.json

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

assets/item/leaf.json

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

assets/item/log.json

@@ -0,0 +1,6 @@
+{
+    "id": 5,
+    "name": "log",
+    "description": "",
+    "texture": "texture/item/block/log.png"
+}

assets/item/sand.json

@@ -0,0 +1,6 @@
+{
+    "id": 4,
+    "name": "sand",
+    "description": "",
+    "texture": "texture/item/block/sand.png"
+}

assets/item/snowy_grass_block.json

@@ -0,0 +1,6 @@
+{
+    "id": 8,
+    "name": "snowy_grass_block",
+    "description": "",
+    "texture": "texture/item/block/snowy_grass_block.png"
+}

assets/item/stone.json

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

assets/item/water.json

@@ -0,0 +1,6 @@
+{
+    "id": 7,
+    "name": "water",
+    "description": "",
+    "texture": "texture/item/block/water.png"
+}