only respect commits against `main` for reference

[bpmct]

We have a pretty strict commit format on coder/coder. However, aicommit is still doing non-ideal titles: coder/coder@5c4ec96

Alternatively, I know we could add commits.md but I think the Ai could be adjusted to understand the proper title

[ammario]

I noticed this as well and don't fully understand why the prompt fails to follow style on coder/coder. The COMMITS.md is a good idea in any case as it helps educate non-AI users on style.