chore: update readme

[hurricanenara]

[!NOTE] Updates labels README to use a single enum label (not array), revises allowed categories, and refreshes usage/example.

• README (labels/README.md)
  • Switch representation to a single enum label instead of array labels.
  • Revise allowed categories:
    • Add verified, benign.
    • Remove spam; keep warning, malicious.
  • Update usage instructions and JSON example to {"label": "verified"}.
  • Minor heading and wording adjustments (e.g., singular "Allowed Label").

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