Add flakes list support to flake validators

Summary

Add support for validating multiple flakes using matrix jobs in flake validators.

Changes

• Added flakes option (default [".]) to flake-lock, flake-show, and flake-strict-inputs modules
• All validators now use matrix jobs to validate each flake in the list (similar to omnix/cachix)
• Composite flake module passes flakes list to all sub-modules
• Updated strict-inputs validator to accept flake path as argument
• Updated CI config to validate test flakes alongside main flake
• Added documentation for validating multiple flakes

Benefits

This ensures test flakes have their inputs validated too, catching issues like:

• Unpinned dependencies in test environments
• Outdated flake.lock files in test directories
• Missing or invalid refs in test flake inputs

Example Usage

flake = {
  enable = true;
  flakes = [ "." "./test/my-test-flake" ];  # Validates both flakes
};

This creates matrix jobs that run each validator for every flake in the list.

🤖 Generated with Claude Code