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

default.nix

@@ -0,0 +1,27 @@
+{pkgs}: let
+  inherit (pkgs) lib;
+  templateName = builtins.baseNameOf (toString ./.);
+in {
+  ${templateName} = pkgs.stdenvNoCC.mkDerivation {
+    pname = templateName;
+    version = "0.1.0";
+    src = ./.;
+    dontBuild = true;
+    installPhase = ''
+      mkdir -p $out/share/${templateName}
+      if [ -d src ]; then
+        cp -r src $out/share/${templateName}/
+      fi
+      for f in README.md project.toml flake.nix default.nix shell.nix Makefile .editorconfig .gitignore; do
+        if [ -f "$f" ]; then
+          cp "$f" $out/share/${templateName}/
+        fi
+      done
+    '';
+    meta = with lib; {
+      description = "Template project: ${templateName}";
+      license = licenses.unfreeRedistributable;
+      platforms = platforms.all;
+    };
+  };
+}