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

flatpak/app.flatpak.json

@@ -0,0 +1,22 @@
+{
+    "app-id": "org.example.dotnetdevshell",
+    "runtime": "org.freedesktop.Platform",
+    "runtime-version": "23.08",
+    "sdk": "org.freedesktop.Sdk",
+    "command": "dotnetdevshell-app",
+    "modules": [
+        {
+            "name": "dotnetdevshell-app",
+            "buildsystem": "simple",
+            "build-commands": [
+                "dotnet publish -c Release -o out"
+            ],
+            "sources": [
+                {
+                    "type": "dir",
+                    "path": ".."
+                }
+            ]
+        }
+    ]
+}