Flag the invariant tag question "isn't it"?

**The Error**

A tag question is a short yes/no question for confirmation attached to a statement.
In English there is grammatical agreement between the statement and the tag question:
- It's expensive **isn't it?**
- We're lost **aren't we?**
- Your clever **aren't you?**
- They missed their flight **didn't they?**
- That's not good **is it?**

There are also slang/dialect/casual/informal tag questions in English that don't have agreement:
- It's cold **innit?**
- They're good **yeah?**
- It's that one **right?**

But I think mainly some Indians and South Africans are particularly prone to using "isn't it?" as a universal "invariant" tag question, disagreeing with the verb in the statement:

- Sounded like a Churchill speech, **isn’t it?** 👍

Start by locating Harper’s existing grammar rules and tests for tag questions or subject–verb agreement. Compare the reported invariant “isn’t it?” example with the accepted agreement examples, then add coverage showing the intended diagnostic and verify the relevant test suite passes.

Written by the indexing model from the issue text.