Expose package itself from the flake

Hello,

thanks a lot for putting this useful tool together. One blocker which I found is that flake.nix does not expose the package itself (I'm not able to use your shell I want to include the tool inside my shell where I provide cardano-node myself). Should I open a PR with something like:

```nix
packages.cardonnay = pkgs.python313.pkgs.buildPythonApplication {
pname = "cardonnay";
version = "0.3.4";
src = self;
pyproject = true;

build-system = with pkgs.python313.pkgs; [ setuptools setuptools-scm ];

SETUPTOOLS_SCM_PRETEND_VERSION = "0.3.4";

nativeBuildInputs = with pkgs.python313.pkgs; [ pythonRelaxDepsHook ];

pythonRelaxDeps = [ "setuptools" ];

dependencies = with pkgs.python313.pkgs; [
supervisor click pygments pydantic filelock
];
};
```

Start in flake.nix by comparing the existing shell definition with the proposed packages.cardonnay expression. Done means the flake exposes the package itself so users can include it in a shell without using the provided cardano-node shell; verify the flake evaluates successfully.

Written by the indexing model from the issue text.