Remove Autofac dependency — use reflection for IDay discovery

[Copilot]

Autofac was only used for assembly scanning to discover IDay implementations. Since none have constructor dependencies, a DI container is unnecessary.

• Removed Autofac package reference from AdventOfCode.csproj
• Replaced ContainerBuilder/Resolve in Program.cs with plain reflection:

var days = Assembly.GetExecutingAssembly()
    .GetTypes()
    .Where(t => t is { IsClass: true, IsAbstract: false } && typeof(IDay).IsAssignableFrom(t))
    .Select(t => (IDay)Activator.CreateInstance(t)!);

All 182 tests pass. No other files reference Autofac.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.