Remove Autofac dependency — use reflection for IDay discovery (#53)

* Initial plan

* Replace Autofac with reflection-based assembly scanning

Remove Autofac NuGet dependency and replace the DI container usage in
Program.cs with simple reflection to discover and instantiate IDay
implementations. Since none of the Day classes have constructor
dependencies, a DI container is unnecessary.

Co-authored-by: ianfnelson <205606+ianfnelson@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ianfnelson <205606+ianfnelson@users.noreply.github.com>

Author: Copilot

AdventOfCode/AdventOfCode.csproj

@@ -16,9 +16,6 @@
       <Folder Include="Events\2024\InputFiles\" />
     </ItemGroup>
 
-    <ItemGroup>
-      <PackageReference Include="Autofac" Version="9.0.0" />
-    </ItemGroup>
 
     <ItemGroup>
       <None Remove="Events\2024\InputFiles\1.txt" />

AdventOfCode/Program.cs

@@ -1,24 +1,17 @@
 using System.Diagnostics;
 using System.Reflection;
 using AdventOfCode.Framework;
-using Autofac;
 
-var container = BuildContainer();
-
-var days = container.Resolve<IEnumerable<IDay>>();
+var days = Assembly.GetExecutingAssembly()
+    .GetTypes()
+    .Where(t => t is { IsClass: true, IsAbstract: false } && typeof(IDay).IsAssignableFrom(t))
+    .Select(t => (IDay)Activator.CreateInstance(t)!);
 var day = GetDay();
 Console.WriteLine("Year " + day.Year + " Day " + day.Day);
 var inputPath = $"Events/{day.Year}/InputFiles/{day.Day}.txt";
 DoPart(1, () => day.Part1(inputPath));
 DoPart(2, () => day.Part2(inputPath));
 
-IContainer BuildContainer()
-{
-    var builder = new ContainerBuilder();
-    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly()).AsImplementedInterfaces();
-    return builder.Build();
-}
-
 IDay GetDay()
 {
     if (args.Length == 0)