check_contributors.py is a 650-line single file #1319

Where: scripts/check_contributors.py.

Current gap: Parsing, the checks, the commit-data fetching, --fix, --stub, --identities and the exception loading all live in one file (653 lines), and each new flag has made it longer even after main() was split by mode (#1260).

Suggested approach: Move the gh fetching, the stub and fix writers and the identity check into modules next to it (scripts/contributors_*.py) with the command line staying in check_contributors.py, keeping every test unchanged.

Done when: no module is over 300 lines and the existing tests pass without edits.