Add Nix flake support to the GitHub repo

[diffnull]

To make it easy to install ck in Nix environment I propose to add the Nix flake support to this repo.

[Cody-W-Tucker]

ck is packged in this repo, works nicely on nixos. https://github.com/numtide/llm-agents.nix?tab=readme-ov-file#utilities

[runonthespot]

We don't currently ship a Nix flake but would happily accept a PR for one. The output would want to expose at minimum a packages.${system}.default for the ck binary built from ck-cli. The release pipeline already builds linux-x86_64 / macos-{x86_64,arm64} / windows-{x86_64,aarch64} binaries (see .github/workflows/release.yml) so the flake just needs to mirror those targets.

Happy to review if anyone wants to take this on.