Follow-up for salvaged #88472 (+ #100055 / #99995 / #88998 intent):
- add_contributor.py compares filenames with str.casefold(), the same key
scripts/check-case-collisions.py uses repo-wide, so non-ASCII folds
(ß ~ ss) are caught the way macOS/Windows fold them.
- The KNOWN_CASE_CONFLICTS allowlist is gone: the historical
agent@Agents-Mac-mini.local pair was removed on main (fcdae2cf0b), so the
repo-wide test asserts zero collisions.
- New tests: same-login different-spelling is still refused (the filename
pair is the problem, not the login) and the exact spelling stays
idempotent; casefold vs lower coverage.
Co-authored-by: alfred-amanda <288490622+alfred-amanda@users.noreply.github.com>