Explore attaching composition summary to pull requests

[justinrobots]

Description

We should explore how this information could be attached to PRs or otherwise made available to anyone who is reviewing code for a branch. Mechanically, we could do this with the GK draft system or something like Git notes.

Some things to consider:

• users don't have to compose all commits on a branch, and they could compose groups of commits on a single branch multiple times - how would we collect/store that information for a more comprehensive review?
• Could we have a separate feature that generates this kind of doc at the moment a PR is created?