feat: Flag confusing contraction who's and possessive whose

Common among native speakers and non-native speakers alike:

Whose there? → who's
Who's idea is it? → whose

My browser catches the first one:

List of read-world examples:
- Bill Burr was the only Comedian **who’s** brain wasn’t ruined → ... **whose** brain wasn’t ruined

Start by locating the Harper grammar rules or tests that handle the contractions “who’s” and “whose”; the issue does not name specific files or entry points. Use the supplied example sentences as regression cases, and consider the work complete when the incorrect contraction is flagged and the possessive form is accepted in the corresponding examples.

Written by the indexing model from the issue text.