Poison bot commit messages

We've talked about being able to better reason about commits and PRs. That's really hard, in part because of all the bot activity.

We should consider adding a marker to commits/PRs that should never participate in automated "Feature and fix reports". Many projects auto-generate lists of improvements directly out of GitHub. We do not.

As you can see, we have many bot-generated PRs.

https://github.com/dotnet/runtime/pulls/app%2Fdotnet-maestro

Certainly, we can block-list all the bot accounts.

We could also consider adding other markers for other purposes, like community PRs.