ci: add gemini code assist config (#31512)

Configures gemini to not interact with new pull requests, only review on
demand

Author: mmalerba

.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: false
+    code_review: false
+ignore_patterns: []