feat(nix): Implement nix-fmt hook

[aRustyDev]

Description

Implement a pre-commit hook for formatting Nix files using nixpkgs-fmt or alejandra.

Requirements

• Support multiple formatters (nixpkgs-fmt, alejandra, nixfmt)
• Auto-install formatter if not available
• Format files in-place
• Support configuration for formatter choice
• Check mode (verify formatting without changing files)

Implementation Details

• Location: hooks/nix/nix-fmt.sh
• Language: bash script
• File pattern: *.nix

Testing

• Test with unformatted nix files
• Test with already formatted files
• Test formatter auto-installation
• Test different formatter choices
• Test check mode vs format mode

Configuration in .pre-commit-hooks.yaml

- id: nix-fmt
  name: Format Nix files
  description: Formats Nix files using nixpkgs-fmt or alejandra
  entry: hooks/nix/nix-fmt.sh
  language: script
  files: \.nix$
  args: [--formatter=nixpkgs-fmt]