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/SpawnModelCommand.cs

@@ -0,0 +1,15 @@
+using System.Numerics;
+
+namespace Engine.AI.Commands;
+
+/// <summary>
+/// Spawn a named entity with a Mesh loaded from a model file and a Transform.
+/// </summary>
+public sealed record SpawnModelCommand : AiCommand
+{
+    public required string Name { get; init; }
+    public required string ModelPath { get; init; }
+    public Vector3 Position { get; init; } = Vector3.Zero;
+    public Quaternion Rotation { get; init; } = Quaternion.Identity;
+    public Vector3 Scale { get; init; } = Vector3.One;
+}