chore: remove Nix and the std deployment framework
Remove all Nix tooling and the std (divnix/standard) framework, which are no longer used: - flake.nix / flake.lock and the entire nix/ tree (cardano-services k8s deployments, OCI images, operables, terraform outputs, kubeconfigs) - std workflows (std.yml, std-release.yaml, std-unexploded.yml) and the Nix-based cardano-services deploy workflow - the yarn-plugin-nixify plugin and the generated yarn-project.nix - treefmt.toml (Nix-only formatter config), .envrc / direnv, and the prj-spec .gitignore entries Drop the now-obsolete yarn-project.nix references from the unit-test workflow's "Generated files" step, the Dockerfile COPY layer, the npm version script, and the README config-update instructions. yarn install --immutable remains clean without the nixify plugin. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>