Seed: price calculator with discount tests

Mini project for agentd live testing: three of the five tests fail on main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/Prices.Tests/PriceCalculatorTests.cs

@@ -0,0 +1,36 @@
+using Xunit;
+
+namespace Prices.Tests;
+
+public sealed class PriceCalculatorTests
+{
+    [Fact]
+    public void No_discount_keeps_the_price()
+    {
+        Assert.Equal(100m, PriceCalculator.ApplyDiscount(100m, 0m));
+    }
+
+    [Fact]
+    public void A_full_discount_makes_it_free()
+    {
+        Assert.Equal(0m, PriceCalculator.ApplyDiscount(200m, 100m));
+    }
+
+    [Fact]
+    public void A_quarter_off_200_is_150()
+    {
+        Assert.Equal(150m, PriceCalculator.ApplyDiscount(200m, 25m));
+    }
+
+    [Fact]
+    public void Half_off_80_is_40()
+    {
+        Assert.Equal(40m, PriceCalculator.ApplyDiscount(80m, 50m));
+    }
+
+    [Fact]
+    public void A_discount_above_100_percent_is_refused()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => PriceCalculator.ApplyDiscount(100m, 101m));
+    }
+}