Harper suggests I change the two dashes starting mail signatures to an em-dash

**What got flagged?**
I write mail in Neovim (in a TUI mail client). Harper suggests I change the `-- ` near the end of the file that starts the user's signature to an em-dash. I have no way to add it to a dictionary — not that Harper offers to. I do use em-dashes in general, so I don't want to disable the feature, and I couldn't add it to a workspace dictionary because mail files being composed are in `/tmp/`.

**Why is this incorrect?**
`-- ` is the standard delimiter for an email [signature block](https://en.wikipedia.org/wiki/Signature_block). If I change it my signature won't be recognized.

**Example of correct usage:**
The following file, `mail` filetype in Vim/Neovim, be considered correct by Harper.
```
Hello world.

--
John Smith
```

**Suggested solution**
Perhaps Harper could ignore `-- ` in mail files when it's the only thing on the line?

Start by tracing Harper's handling of the `-- ` line in mail or Neovim input and identify the rule that suggests an em-dash. Confirm that the signature delimiter is ignored only when it is the sole line content, while ordinary em-dash checks remain active; add a regression test for the shown example.

Written by the indexing model from the issue text.