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.

include/Cubed/gameplay/item.hpp

@@ -0,0 +1,14 @@
+#pragma once
+#include <cstdint>
+#include <string>
+namespace Cubed {
+using ItemID = uint16_t;
+
+struct ItemData {
+    ItemID id = 0;
+    std::string name;
+    std::string description;
+    std::string path;
+};
+
+} // namespace Cubed

include/Cubed/gameplay/item_manager.hpp

@@ -0,0 +1,36 @@
+#pragma once
+
+#include "Cubed/gameplay/item.hpp"
+
+#include <filesystem>
+#include <string_view>
+#include <tbb/concurrent_hash_map.h>
+namespace Cubed {
+class ItemManager {
+public:
+    ItemManager();
+    void init();
+    static ItemManager& instance();
+
+    static const ItemData& get(std::string_view key);
+    static const ItemData& get(ItemID id);
+
+    const ItemData& get_item_data(std::string_view key) const;
+    const ItemData& get_item_data(ItemID id) const;
+
+private:
+    using ItemMap = tbb::concurrent_hash_map<ItemID, ItemData>;
+    using acc = ItemMap::accessor;
+    using cacc = ItemMap::const_accessor;
+
+    using IDMap = tbb::concurrent_hash_map<std::string_view, ItemID>;
+
+    void add(const std::filesystem::path& path);
+
+    ItemMap m_map;
+
+    IDMap m_id_map;
+
+    static constexpr ItemData EMPTY;
+};
+} // namespace Cubed