Upgrade to horizon-flake-parts 0.16.0

Summary

• Migrate from flake-parts + import-tree to nixica 0.1.0
• Update horizon-flake-parts from 0.10.0 to 0.16.0
• Remove direct werrorwolf input (now provided via horizon-flake-parts.flakeModule)
• Convert all modules to local: pattern
• Add strict-inputs compliant refs for horizon-core
• Follow nixpkgs from horizon-flake-parts
• Update .gitlab-ci.yml remote template from 0.9.0 to 0.19.0

Closes #7 (closed)