chore(docs): update contributing guidelines to include omnibor

[20jasper]

While working on #2223, I found omnibor is required to run integration tests. This PR includes a script to install required testing tools

However, there are a few things I'd like to clear up before going further

doubts

README/CONTRIBUTING.md overlap

Both the README.md and CONTRIBUTING.md have information about contributions—some is overlapping and some is not. Would y'all be interested in any of these options?

1. move contributing info to CONTRIBUTING.md and link in the README
2. move contributing info fully to the README

Nix

Would y'all prefer updating the devenv.nix be

• made into a separate issue
• note the current failures are expected when using the devenv.nix

As of now, the integration test using omnibor fails in the nix devenv, and it seems like not an insignficant amount of work to add omnibor to the devenv.nix, though of course I could be missing something obvious since this is the first time I've used nix

From what little I know of nix, running an install script for something without a deterministic lockfile is not the preferred way to do things. For reference, omnibor-rs does not provide a Cargo.lock. I have not checked if there is another deterministic build for this beyond directly downloading the artifacts

Troubleshooting

I was unable to install omnibor via the devenv.nix

It did not seem that omnibor was already available to use like cargo-insta was https://search.nixos.org/packages?channel=25.11&query=omnibor-cli

Had trouble locally installing nix, so ran a dockerfile with the following configuration

FROM nixos/nix 

RUN nix-env --install --attr devenv -f https://github.com/NixOS/nixpkgs/tarball/nixpkgs-unstable

COPY . /workdir
WORKDIR /workdir

ENTRYPOINT [ "bash" ]

After dropping into the shell with devenv shell and running tests via cargo test, reproduced the integration test failure

test axolotlsay_basic ... FAILED

I got the git hash from the omnibor releases chore: Release omnibor-v0.7.0 · omnibor/omnibor-rs@80ff804 · GitHub

As mentioned above, no Cargo.lock complicates building from source

It seems possible to set different download URLs per OS and architecture but wanted to check in before spending too much time here