feat(devenv): Add gemini-cli

Adds the `gemini-cli` package to the development environment to make
it available in the shell.

Fixes #6

devenv.nix

@@ -1,5 +1,7 @@
 { pkgs, lib, ... }:
 {
+  packages = with pkgs; [ gemini-cli ];
+
   git-hooks.hooks = {
     deadnix.enable = true;
     nixpkgs-fmt.enable = true;