False positive: 'PR' and username '@xxx' flagged

## Description

Example:
`Thank you for the PR @hippietrail`

Both PR and the username are flagged. I would expect that:

- `PR` is being recognised as a valid abbreviation (both Pull Request and Public Relations are common)
- whatever comes in a single token starting with @ is not being checked for correctness

## Screenshots

Reproduce the example text `Thank you for the PR @hippietrail}` and inspect the grammar-checking path that flags the abbreviation and the @-prefixed token. Done means `PR` is accepted as a common abbreviation and the single-token username is skipped without producing either diagnostic; add or update regression coverage if the project’s existing test structure identifies a suitable location.

Written by the indexing model from the issue text.