Add learning hour: Show, Don't Tell — using examples with coding agents

Exercise project with Order domain and NUnit tests demonstrating
the 'show, don't tell' pattern for guiding coding agents.

- 1 refactored test using custom matcher (the example)
- 5 unrefactored tests with multiple asserts (the practice)
- OrderStateConstraint custom matcher for NUnit 3

exercises/show-dont-tell/src/OrderDomain/OrderLine.cs

@@ -0,0 +1,19 @@
+namespace OrderDomain;
+
+public class OrderLine
+{
+    public decimal UnitPrice { get; }
+    public int Quantity { get; }
+    public bool HasDiscount { get; private set; }
+
+    public decimal Value =>
+        HasDiscount ? UnitPrice * Quantity * 0.8m : UnitPrice * Quantity;
+
+    public OrderLine(decimal unitPrice, int quantity)
+    {
+        UnitPrice = unitPrice;
+        Quantity = quantity;
+    }
+
+    public void ApplyDiscount() => HasDiscount = true;
+}