Flag "resumptive pronouns" is relative clauses

Maybe best illustrated by an example:
> have this WhatsApp chat that I've slowly been putting **it** together over the years

Here's the Wikipedia article on the term: [resumptive pronoun](https://en.wikipedia.org/wiki/Resumptive_pronoun)

Start with the issue's example and the linked Wikipedia definition of resumptive pronouns. Trace how Harper represents and reports grammar patterns, then identify where relative-clause checks are implemented. Done means the example is flagged as a grammar issue without introducing regressions in related cases.

Written by the indexing model from the issue text.