build runs on push to main and every PR including forks. Same model as
the test/lint/typecheck workflows: read-only token, no secrets injected,
runs scripts/update-nix.sh + nix build to validate. No commit, no push.
update-hash runs only on push to main. Mints the paseo-ai App token,
re-runs the update + build, and commits the refreshed hash via the
ruleset bypass. The default GITHUB_TOKEN no longer pushes anywhere.
Rename file to nix.yml since it's no longer just a build job.
Mohamed Boudra