COMMIT_EXCEPTIONS and UNLISTED_AUTHORS are Python dictionaries in the script #1270

Where: scripts/check_contributors.py, CONTRIBUTING.md.

Current gap: Recording that a contributor's commits are uncountable, or that an author is deliberately unlisted, needs a code change to the script (and a reason string in Python), although the entries are data a maintainer edits.

Suggested approach: Move both lists to scripts/contributors-exceptions.json with a handle, a reason and which check they apply to, read them at start-up and update the CONTRIBUTING section.

Done when: an entry added to the JSON file is honoured without touching the script, covered by a test with a temporary file.