Introduce Nix to the project

[jaredmontoya]

What this Pull Request (PR) does

Don't run extract_wisdom or summarize on this

I am the maintainer of fabric-ai package in the central NixOS repository called nixpkgs

Earlier I created a PR #1088, but to make development a lot more comfortable for myself and other Nix users who decide to contribute to fabric I also wrote this Nix flake which will bring benefits of the Nix ecosystem to fabric.

It improves developer experience and lets people unclutter their systems. Flake maintenance is not resource intensive at all if you know what you are doing, so any Nix user can create a PR if flake.lock needs updating.

What does this bring to the project:

• Reproducibility, if you use nix build to build fabric the build will be binary level reproducible on any system because everything is pinned in the flake.lock including the compiler.
• Development shell, this flake provides a development shell configuration that can be activated automatically using direnv when entering the project root. It includes everything that you will need to develop the project including the go compiler, go tools, gomod2nix,
  and the helper script update that basically executes go mod -u && go mod tidy && gomod2nix generate. Everything in this flake is pinned using flake.lock which is checked into git so you also get a history of ALL your dependencies including the compiler, not just go modules.
• This essentially means that even in a lot of years when old fabric code is incompatible with the new go compiler version you can just revert to the old git commit and automatically get the old compiler version and you will be able to build old versions of fabric with no problems.
• The package provided by this flake.nix means that NixOS users can directly install fabric from your git repository and any of it's branches or tags bypassing the fabric-ai package the central nixpkgs repository that is updated a lot less frequently.
• nix fmt formats all go files codebase with gofmt and all nix files with various nix formatters. It is possible to add yamlfmt.enable = true; to treefmt.nix and then it will also format yaml files, basically it's a tool to format any text files in the project. nix flake check can be used to check if all files are formatted properly. In the future I probably will open a PR that adds goimports-reviser support to treefmt-nix so it will also be able to sort imports.

If you don't know what Nix is and don't use it:

• You can just ignore all files in this PR and use your own compiler like you did before even with those files present. Nothing changes for you personally.
• In the future, if you need to build an old version of fabric with an old version of the compiler instead of having only one option you will have two options:
• • Find out which version of the go compiler is needed for that commit, find it's package somewhere, install it, build the commit you want.
• • Install Nix, checkout the needed commit, run either nix build for a production package or nix develop to enter a devshell with the needed compiler and run go build

What to keep in mind:

• Unfortunately if you run go get -u && go mod tidy or modify go.mod in any other way and then don't run gomod2nix generate after that it will desync go.mod and gomod2nix.toml which can lead to unexpected results when using nix buildor installing the nix package, normal builds will not be affected.
• This problem can be mitigated by creating a github workflow that runs gomod2nix gnerate on all commits so that if go.mod is changed in any of them gomod2nix.toml is automatically synced.
• Another solution might be to just install gomod2nix and remember to run it whenever you change go.mod but you will have to remind this to everyone who creates a PR that requires changing go.mod

[jaredmontoya]

For Nix users: If you want to get the context length fix from #1088 before it is even merged you can do this by adding an overlay like this:

inputs = {
  fabric = {
        url = "github:jaredmontoya/fabric";
        inputs = {
          nixpkgs.follows = "nixpkgs";
        };
      };
};

    nixpkgs.overlays = [
      (self: super: {
        fabric-ai = inputs.fabric.packages.${system}.default;
      })
    ];