CI: add config.yaml for gemini code review

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`

.gemini/config.yaml

@@ -0,0 +1,12 @@
+# Config for the Gemini Pull Request Review Bot.
+# https://github.com/marketplace/gemini-code-assist
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: true
+    code_review: true
+ignore_patterns: []