Add interactive REPL with helpers and improve package management

- Add Node.js to devshell for mature, stable REPL
- Create .replrc.js with println() and dir() helpers
- Add 'just repl' command with auto-loaded helpers
- Add 'just add-dev' and 'just remove' for package management
- Update README with complete command reference and feature list

nix/modules/devshell.nix

@@ -21,6 +21,7 @@ in {
       nativeBuildInputs = with pkgs;
         [
           bun
+          nodejs
           eslint
           just
           prettierd