feat: Flag wrong number agreement between subject and verb

This is usually a mistake made by English learners or by native speakers overlooking results of rearranging wording while editing:

- What **does** **the specs** say? → What **do** the **specs** say?
- The **characters** used **is**: → The **characters** used **are**:

On occasion it might be the noun that's wrong rather than the verb:

- → What **does** the **spec** say?
- → The **character** used **is**:

Sometimes it could be either:
- To me, the load bearing **factor were** the expectations from how open source is typically done ...
- → the load bearing **factor was** ...
- → the load bearing **factors were** ...

Sometimes the writer is confused by words between the inflected noun / head of the noun phrase, and the verb.

The issue provides example sentences but names no files, tests, or entry points. Start by locating the existing grammar-rule handling for subject–verb agreement, then determine how intervening words and noun-versus-verb ambiguity should be handled. Done means the listed examples are diagnosed correctly.

Written by the indexing model from the issue text.