Standardize lts/ghc-9.4.x structure

Summary

This MR standardizes the lts/ghc-9.4.x branch to use a modular nix/ folder structure with flake-parts import-tree.

Changes

• Replaced inline flake configuration with modular nix/ directory structure
• Created four flake-parts modules:
  • nix/systems.nix - Dynamic system configuration via horizon-supported-systems v1.1.0
  • nix/ci.nix - CI configuration using gitlab-ci-omnix v0.4.0
  • nix/treefmt.nix - Formatting tools (deadnix, nixpkgs-fmt, statix)
  • nix/ghc.nix - Pre-built GHC 9.4 package configuration
• Updated flake.nix to use import-tree pattern
• Simplified .gitlab-ci.yml to use remote include (gitlab-ci v0.3.0)

Validation

All local checks passed:

• nix flake lock ✅
• nix fmt ✅ (0 files changed)
• om ci run ✅ (🥳 Success!)

Dependencies

This MR brings the branch in line with 9.6.x, 9.12.x, and 9.14.x which already use this pattern.