Use haskell-make-package-set.flakeModule for lts/ghc-9.6.x

Summary

Convert horizon-build-packages lts/ghc-9.6.x to use haskell-make-package-set.flakeModule for consistent package set building across all Horizon package sets.

Changes

Infrastructure

• Add haskell-make-package-set.flakeModule
• Add horizon-supported-systems for standardized system definitions
• Add gitlab-ci-omnix for CI integration
• Add treefmt-nix for code formatting
• Use import-tree for modular flake-parts configuration

Directory Structure

Created modular nix/ directory:

• nix/systems.nix - horizon-supported-systems integration
• nix/ci.nix - gitlab-ci-omnix integration
• nix/haskell.nix - haskell-make-package-set configuration
• nix/treefmt.nix - code formatting (deadnix, statix, dhall, nixpkgs-fmt)

Cleanup

• Remove develop/ folder (use remote tools instead)

Input Updates

• Update gitlab-ci-omnix: 0.4.0 → 0.5.0
• Update flake-parts, treefmt-nix to latest
• Alphabetize inputs

Package Set

The package set remains minimal (3 packages: cabal-doctest, hscolour, jailbreak-cabal) and serves its bootstrapping purpose unchanged.

Testing

• ✅ nix flake check passes
• ✅ All packages build successfully
• ✅ Treefmt formatting works

Notes

• horizon-hoogle is intentionally NOT included because this minimal bootstrap set doesn't contain the hoogle package itself
• Treefmt configuration matches horizon-ghc pattern for consistency

🤖 Generated with Claude Code