Rename dev/ to ci/, add ci/dev partition options to mkFlake

Summary

• Rename dev/ → ci/ for CI partition (gitlab-ci input)
• dev/ reserved for consumer overrides (devShells, no fallback)
• mkFlake options: ci.flake, ci.modules, dev.flake, dev.modules
• CI partition falls back to nixica's own ci/ — consumers get CI for free
• treefmt-nix stays in root inputs
• Renamed nixica-dev.nix → nixica-partitions.nix
• nixica-ci published via importAndPublish with discover for CI jobs
• nixica-supported-systems module: nixica.systems.supported (default x86_64-linux)
• Bump gitlab-ci to 0.20.2
• flake.lib option moved into modular
• Removed separate lib.nix