feat: Step 5 AI agent command bridge

- Add new Engine.AI project referenced by the App.
- Define JSON command model: spawn_model, set_transform, delete_entity, list_entities.
- Implement AiCommandProcessor with System.Text.Json and custom Vector3/Quaternion converters.
- Wire processor into Program.cs; demo commands spawn and move a second cube.
- AI commands are executed against the live Flecs world and visible in the next frame.

src/Engine.AI/Commands/DeleteEntityCommand.cs

@@ -0,0 +1,9 @@
+namespace Engine.AI.Commands;
+
+/// <summary>
+/// Delete an entity by name.
+/// </summary>
+public sealed record DeleteEntityCommand : AiCommand
+{
+    public required string Name { get; init; }
+}