Renovate horizon-core to use horizon-flake-parts 0.10.0 (ghc-9.14.x)

Updates horizon-core to use horizon-flake-parts 0.10.0 which provides integrated modules for CI, formatting, and systems configuration.

Changes

• Add horizon-flake-parts 0.10.0 input (includes haskell-make-package-set)
• Remove old inputs: gitlab-omnix-ci, horizon-supported-systems, haskell-make-package-set
• Remove old config files: nix/default.nix
• Remove inline flake configuration (was duplicate outputs)
• Add nix/configuration.nix with horizon module configuration
• Add nix/horizon.nix with preserved haskell.packageSet configuration
• Enable horizon.ci.horizon-gen-nix for validation
• Enable horizon.formatting with configuration/* and modifiers.nix excluded
• Keep .gitlab-ci.yml as required GitLab CI entry point
• Remove develop/ flake (use direct nix run URLs instead)
• Update README to match horizon-build-packages style
• Update all flake inputs to latest versions