* fix(nix): switch nixpkgs input from nixos-24.11 to nixos-unstable
nixos-24.11 reached EOL on 2025-06-30. For a dev tool, tracking a
frozen release branch causes dependency versions to go stale.
nixos-unstable provides rolling updates and is the conventional
choice for development packages.
* docs(website): update nix flake example
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Co-authored-by: sk <sk@mercury>