Add 'drop' command to remove items from inventory

Players can ake items but cannot drop them. A drop command would allow placing inventory items back into the current room.

**Acceptance criteria:**
- Add CommandType.Drop to the enum
- CommandParser recognizes drop
- GameEngine.HandleDrop() removes item from inventory, adds to current room's RoomItems
- Non-droppable items (if any) should be handled gracefully
- Output confirmation message in green

先找出 CommandType enum、CommandParser，以及 GameEngine 中現有的 inventory 和 room-item 處理邏輯。以 take 流程為指引，接著確認 drop parsing、從 inventory 移除、放置到目前房間、不可丟棄物品的處理，以及綠色確認輸出是否都符合驗收標準。

由索引模型根據 Issue 內容生成。