Scaffold a reproducible Nix devShell for a project. Detects project language(s) from sentinel files (go.mod, package.json, pyproject.toml, Cargo.toml), generates a flake.nix with appropriate packages and tooling, creates an .envrc for direnv integration, and verifies the shell builds. Use when onboarding to a project that needs reproducible dev dependencies, adding Nix to an existing repo, or setting up a new project with pinned tooling.
