Add initial project structure with Rust Nix flake template

- Create .envrc for Direnv configuration
- Add .pre-commit-config.yaml for pre-commit hooks
- Include CONTRIBUTING.md for contribution guidelines
- Initialize Cargo.toml for Rust package configuration
- Update LICENSE to link to full license text
- Create Makefile for build, run, test, and clean commands
- Revise README.md for project description and usage instructions
- Implement default Nix flake configuration in flake.nix
- Add Flatpak manifest for packaging
- Set up project metadata in project.toml
- Create shell.nix for legacy shell support
- Add skeletal Rust application in src/main.rs
- Include sample test in tests/sample_test.rs

shell.nix

@@ -0,0 +1,15 @@
+{
+  pkgs ? import <nixpkgs> { },
+}:
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.rustc
+    pkgs.cargo
+    pkgs.rustfmt
+    pkgs.clippy
+    pkgs.rust-analyzer
+  ];
+  shellHook = ''
+    echo "[shell.nix] Legacy shell for Rust dev. Use 'nix develop' for full flake support."
+  '';
+}