* Rewrite agones-bot, commit to Agones repo

This is a direct rip-off of the Quilkin
https://github.com/googleforgames/quilkin/tree/main/build/ci/github-bot
with some name changes. I also re-adapted the template to match our
existing format.

To see it in action:
* success: https://github.com/googleforgames/agones/pull/3917#issuecomment-2253159106
* failure: https://github.com/googleforgames/agones/pull/3917#issuecomment-2253159103

It is already deployed, so this is a commit-after-the-fact.

* Fix lint; fix dry run bug

* Skip on cancel as well

* Address review comments

* Add link to agones-bot in CONTRIBUTING