Release 0.12.0: Add deduplication for systems and flakes options

Summary

This release introduces improvements to reduce configuration duplication across CI modules and uses standard nixpkgs systems list.

Changes:

• Use nixpkgs.lib.systems.flakeExposed for systems list
• Add systems module for deduplicating build systems configuration
• Add automatic flake validation to build modules
• Improve flakes option defaults to reduce duplication

New systems module:

• Adds gitlab-ci.systems module that provides a shared systems option
• omnix and cachix modules now default to config.systems if available
• Falls back to current system if systems module not imported

Auto-enable flake validation:

• omnix and cachix now enable flake validation by default
• Only applies if flake module is also imported
• Can be disabled with flake.enable = false

Smart flakes defaults:

• omnix.flakes and cachix.flakes default to flake.flakes if available
• Falls back to [ "." ] if flake module not enabled
• Eliminates duplication when same paths used across modules

Example configuration simplification:

Before:

omnix.systems = [ "x86_64-linux" ];
omnix.flakes = [ "." "./test" ];
flake.flakes = [ "." "./test" ];

After:

systems = [ "x86_64-linux" ];
flake.flakes = [ "." "./test" ];
omnix.enable = true;

Test plan

• Verify flake evaluation with nix flake show
• Test that omnix and cachix inherit systems and flakes correctly
• Verify backward compatibility with explicit configurations

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