Nix DevShell Init

Scaffold a reproducible flake.nix and .envrc for a project based on detected languages and tooling needs.

What this skill does

1. Detects project language(s) from sentinel files in the working directory.
2. Selects the appropriate devShell packages for each language.
3. Generates a flake.nix with a devShells.default output.
4. Creates an .envrc with use flake for direnv integration.
5. Adds .direnv/ to .gitignore (direnv cache directory).
6. Verifies the generated flake evaluates cleanly.

Language detection