chore: add full nix development support using go-overlay

[Ahhhh-man]

This introduces a flake that pulls in go-overlay to provide a Go toolchain matched from go.work, the buildGoApplication helper used to package the CLI, and the govendor tool needed to keep vendoring reproducible. Using go-overlay lets the project build with standard Go module metadata instead of relying on nixpkgs' older Go packaging path.

See https://github.com/purpleclay/go-overlay

closes #179

Downstream example:

{
  description = "Example consumer of jsonschema/jv";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";

    jsonschema = {
      url = "github:santhosh-tekuri/jsonschema";
      inputs.nixpkgs.follows = "nixpkgs";
      inputs.flake-utils.follows = "flake-utils";
    };
  };

  outputs = { self, nixpkgs, flake-utils, jsonschema, ... }:
    flake-utils.lib.eachDefaultSystem (
      system: let
        pkgs = import nixpkgs { inherit system; };
      in
        with pkgs; {
          devShells.default = mkShell {
            buildInputs = [
              jsonschema.packages.${system}.default
            ];
          };
        }
    );
}