This PR attempts to break `codex-rust-review.md` into sections so that
it is easier to consume.
It also adds a healthy new section on "Assertions in Tests" that has
been on my mind for awhile.
The goal of this change is to try an experiment where we try to get AI
to take on more of the code review load. The idea is that once you
believe your PR is ready for review, please add the `codex-rust-review`
label (as opposed to the `codex-review` label).
Admittedly the corresponding prompt currently represents my personal
biases in terms of code review, but we should massage it over time to
represent the team's preferences.
