Add Alex Hick's Lean PR Summary/Review Actions

At RWC, Quang Dao from [Arklib](https://github.com/Verified-zkEVM/ArkLib) had a slide on how they use LLMs to provide additional input on PRs and that it's been (surprisingly?) helpful.
Specifically, they use [alexanderlhicks/lean-summary-workflow](https://github.com/alexanderlhicks/lean-summary-workflow/) and [alexanderlhicks/lean-review-workflow](https://github.com/alexanderlhicks/lean-review-workflow/tree/main). They summary one we could probably use as-is, the review one strongly assumes that there is some kind of natural-language specification, which might not be true for us but can probably be adapted easily.

It's not hard to see that, if we want to quickly expand to cover multiple use cases, we'll run into a serious bottleneck with @math-fehr's reviewing capacity, and while this clearly won't replace human review, it can be useful to (a) provide a layer of "pre-review" with issues to fix before actually requesting human review, and (b) let human reviewers focus on conceptual/design/etc level feedback rather than boring basics.

.... discuss! ;)