Replace python presubmit with better system

This is more of a "nice-to-have".

The current presubmit tools have worked great for a while, but they're difficult to maintain since they're precariously configured in the `tools` system. I think ideally this should be migrated to a different system that is more robust or externally maintained. Ideally something that is easier to share with other projects in a reasonable way as well.

## Acceptance Criteria

* Git hooks logic lives outside this repo, but is easy to configure within this repo.

Start by inspecting the current presubmit configuration in the `tools` system and how its Git hooks are wired. Compare externally maintained systems that can be configured from this repository; done means the hook logic lives outside the repo while remaining easy to configure here.

Written by the indexing model from the issue text.