similarity compare fails on CLI using `-set1-file` & `-set2-file` args with complaint about missing `@`

I'm running

```
runoak -i phenio.db similarity -p i –set1-file my_mp_terms.txt –set2-file all_hp_terms.txt -O csv
```
and it's failing with `ValueError: '@' is not in list`

It's working successfully with:
```
runoak -i semsimian:../../semsimian-server/phenio.db similarity -p i .idfile high_confidence_phenotyptes.tsv @ .idfile all_hp.tsv -O tsv
```

(apparently just a small CLI bug)

Start with the similarity CLI argument parsing for `-set1-file` and `-set2-file`, comparing it with the working positional syntax that includes `@`. Reproduce the reported command and confirm it completes without the `ValueError: '@' is not in list` error.

Written by the indexing model from the issue text.