add architecture skeleton

GymLogAI.Core/Entities/Exercise.cs

@@ -0,0 +1,12 @@
+namespace GymLogAI.Core.Entities;
+
+public class Exercise
+{
+    public Guid Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public float[]? Embedding { get; set; }
+    public DateTime CreatedAtUtc { get; set; }
+
+    public List<WorkoutExercise> WorkoutExercises { get; set; } = [];
+}