Need to flag plural subject vs singular present tense verb agreement error

Some real-world grammar problems from our Discord that Harper doesn't flag:

> Web page**s** seem**s** to work pretty well so far.

"**pages seems**" should be either "**pages seem**" or "**page seems**"

Agreement is hard because there are so many factors and nouns, verbs, and other POSes can all be phrases as well as verbs.

But I'm starting to think that we could tackle agreement problems piecemeal as we encounter the most common ones, by using merged linters of small parts that handle one focused case each.

No source file or test is identified in the issue. Start by reviewing Harper's existing merged linters and how they represent agreement checks, using the reported "pages seems" example as the initial case. Done means the checker flags the plural-subject/singular-verb mismatch while accepting "pages seem" and "page seems".

Written by the indexing model from the issue text.