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>

src/Prices/PriceCalculator.cs

@@ -0,0 +1,21 @@
+namespace Prices;
+
+/// <summary>Turns a list price into what the customer actually pays.</summary>
+public static class PriceCalculator
+{
+    /// <summary>Applies a percentage discount to a price, e.g. 25 for a quarter off.</summary>
+    public static decimal ApplyDiscount(decimal price, decimal discountPercent)
+    {
+        if (price < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(price), "A price cannot be negative.");
+        }
+
+        if (discountPercent is < 0 or > 100)
+        {
+            throw new ArgumentOutOfRangeException(nameof(discountPercent), "A discount is between 0 and 100 percent.");
+        }
+
+        return price - discountPercent;
+    }
+}

src/Prices/Prices.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>