Reproducible Dev Environments with Nix and direnv

I still remember the morning I joined a new project and spent three hours fighting with Node.js versions. The README said "Requires Node 18," but my system had 20, and nvm was having one of its moods. By the time I got the environment working, I'd forgotten why I wanted to contribute in the first place. That's when I realized: your development environment shouldn't be a puzzle you solve before you can write code. The Chaos of "Just Install It" Traditional development setup follows a familiar pattern. Clone the repo, read the README, install the right language version, install dependencies, realize you need a specific database version, install that, configure environment variables, discover the project needs an older version of some CLI tool, fight with your package manager, and eventually — if you're lucky — run the application. The problems multiply when you switch between projects. That Node 18 project conflicts with your Node 20 side project. The Python 3.9 dependency breaks your sy