So far, most of the AI code review tools I've seen in the wild have been
pretty lackluster. I figure we should give the new Gemini Code Assist
variant a try: https://github.com/marketplace/gemini-code-assist/.
This PR adds an initial yaml config for it.
Relevant commands (as comments on the OP PR):
* `/gemini review` to perform a code review
* `/gemini summary` provides a code summary
* comment like normal tagging `@gemini-code-assist`
* `/gemini help`
