refactor: nodes config field

[cmdoret]

Proposed Changes

Replace the subjects field by nodes in rules config. See #54

Types of Changes

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• A bug fix (non-breaking change which fixes an issue).
• A new feature (non-breaking change which adds functionality).
• A breaking change (fix or feature that would cause existing
  functionality to not work as expected).
• A non-productive update (documentation, tooling, etc. if none of the
  other choices apply).

Checklist

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  CONTRIBUTING
  guidelines.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added the necessary documentation (if appropriate).

[supermaxiste]

Looks good to me 👍