🚀 Feature: --dry-run flag to report the detected formatter without formatting

> [pending] will clean up this text

### Feature Request Checklist

- [x] I have pulled the latest `main` branch of the repository.
- [x] I have [searched for related issues](https://github.com/JoshuaKGoldberg/formatly/issues?q=is%3Aissue) and found none that matched my issue.

### Overview

Followup to #123, which asked for both printing the detected formatter and a `--dry-run` flag. #570 implements the printing; this is the remaining flag, which would report which formatter was detected and what would run, without formatting anything.

### Additional Info

Original ideas from @rubiesonthesky in #123.

The issue names no files, tests, or entry points. Start by tracing the CLI argument handling and formatter-selection flow, then verify that the new flag reports the detected formatter and planned action without changing files; add or run coverage for the dry-run behavior.

Written by the indexing model from the issue text.