Document and settle on a commit message convention

Currently it seems that we use different conventions for public commit messages in this repository:
- @cdleary tend to use `[Subsystem:Sub subsystem][downcase(Type of change)] Message`
- @proppy tend to use `subdirectory: message`
- other contributors tend to use `Message`

In general `Message` start with an action verb like `Add`, `Fix`, `Update` or `Improve`.

It would be nice to settle on a unified convention like @cdleary's or re-use well established one like https://www.conventionalcommits.org/.

That would also help w/ things like #1380 and can also be enforced with pre-submit github actions.