CI does not enforce formatting/lint (fourmolu, cabal-fmt, hlint) #72

Gap found while working through the Month 2 roadmap item 'Update CI'.

flake.nix defines a pre-commit flake check (ormolu, cabal-fmt, nixfmt) intended to run locally as a git pre-commit hook, and CONTRIBUTING.md states: 'They are run automatically as pre-commit hooks, but CI will run them again and expect that to be a no-op.'

That is currently not true: .github/workflows/haskell.yml only builds/tests with cabal, it never runs nix flake check or any lint/format tool. So a PR with unformatted code or hlint violations can merge as long as it builds and tests pass, contradicting CONTRIBUTING.md and silently relying on contributors having pre-commit hooks installed locally.

Note: the hook already uses ormolu (ormolu.enable = true in flake.nix), not fourmolu — fourmolu.yaml at the repo root is dead config, never actually wired to the hook.

Proposed fix:

• Add a CI job that runs the existing checks.pre-commit flake output (nix flake check) so formatting is enforced without requiring local hooks
• Add an hlint CI step (hlint is already available in the nix devShell, per .hlint.yaml)
• Update CONTRIBUTING.md if the enforcement mechanism changes