Need to flag indefinite article vs noun number agreement error

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

> ... but I have included **a** notic**es** on my landing ...

"**a notices**" should be either "**a notice**" or "**notices**" (or "**some notices**")

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.