Better commit messages

Currently the commit messages of the bot commits aren't very descriptive. They have a generic name and a reference to a commit that may or may not be the commit that triggered the build. There are a few ways these could be improved. Two cases need to be considered:

1. Builds triggered from within this repo, e.g. by pushing to main or through manual triggers.
2. Builds triggered by the tracks, e.g. from changes to the `.appends` dir.

The first case is fairly simple to handle:

- on `push`: Use the commit message of the commit that triggers it.
- on `workflow_dispatch`: Add an input for the commit/PR title and message.

Perhaps with a standard addendum that points to the doc page describing org-wide-files and that it will be automerged.

The second case is more complicated. We could use the commit message of the commit that changed `.appends`. Either by making it part of the `repository_dispatch` payload, or by adding the commit hash to the payload and fetching the message from the GitHub API. However, since we're now dealing with untrusted data, we might have to sanitise it or add proper safeguards. Currently the repository dispatch doesn't contain any info other than the track that triggered the build, which is not changeable by users.

---

Personally, I don't particularly care for the commit history of files that are changed upstream and I don't expect there to be all that many changes in general, so I don't think this is a priority for now. However, I know some maintainers might disagree :D

---

cc @ee7