Add initial project structure with configuration and sample files

- Create .envrc for Direnv configuration
- Add .pre-commit-config.yaml for pre-commit hooks
- Include CONTRIBUTING.md with contribution guidelines
- Implement Makefile for build, run, and clean commands
- Set up Program.cs with a basic Hello World message
- Define SampleApp.csproj for .NET project configuration
- Add agents.md for coding agent instructions
- Create default.nix for Nix flake setup
- Initialize deps.nix for NuGet dependencies
- Configure Flatpak app with app.flatpak.json
- Implement install-templates.sh for installing .NET templates
- Define project.toml for project metadata
- Set up shell.nix for development shell environment
- Add SampleTest.cs for basic unit testing

Program.cs

@@ -0,0 +1,9 @@
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine("Hello from .NET skeletal app. Edit Program.cs to get started!");
+    }
+}