Fix JSON validation to require the cows directory

[floze-the-genius]

Summary

• only treat .json files under cows/ as cow descriptions
• add regression coverage for root-level and unrelated JSON files
• run the validator tests in the pull request workflow
• let validator-only maintenance PRs pass after tests instead of requiring a cow payload

Closes #49.

Testing

• npm ci
• npm test (5 tests)
• node --check .github/scripts/validate-pr.js
• node --check .github/scripts/validation-helpers.js
• node --check .github/scripts/validation-helpers.test.js
• git diff --check

AI assistance disclosure

This fix, its regression tests, and this pull request description were prepared with OpenAI Codex under the contributor account owner’s direction. The submitted change was validated locally with the commands above.

[cla-assistant]

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[github-actions]

🧪 PR Validation Results for #52

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❌ No JSON file found! Please include your .json file.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[oohwooh]

Thanks so much for the contribution! I especially appreciate that you disclosed your AI use. It is very helpful context to have as a maintainer.

I am busy attending a conference this/next week but will do my best to review this when I can!