[gemini] add configuration for Gemini code review (#12619)

This commit adds the .gemini/config.yaml file to configure the Gemini
code review tool. The configuration includes settings for review
severity, comment limits, and automated review triggers for pull
requests.

.gemini/config.yaml

@@ -0,0 +1,10 @@
+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
+    include_drafts: true