Migrate commit conventions

Ages ago, I started using emoji-keys as an easy-to-identify sorting mechanism for commits. It was nicer than "conventional commits", which bastardizes titles by throwing too much context in an area where 50 characters is the _actual_ convention -- however the impact is that it forces pinning commits into categories that don't actually fit well. This is a similar problem that conventional commits face as well.

I'd like to migrate to using Git trailers effectively instead.

Specifically:

* `Change-Category` for major / minor / patch (enforced as part of presubmit, and CI)
* `Component` for identify which specific component(s) were modified
* `Fixes` for referencing tracking systems (GitHub Issues currently, but in the future this could be Jira)

## Acceptance Criteria

* The commit convention is changed over
* Convention is upheld on precommit

Start by locating the current emoji-key or commit-convention checks, along with the precommit, presubmit, and CI configuration. Trace where commit messages are validated, then confirm that Change-Category, Component, and Fixes are enforced; done means the convention is migrated and upheld on precommit.

Written by the indexing model from the issue text.