gemini: Disable auto PR codereviews for now

I'm assuming it's still possible to trigger commands like `/gemini review` manually.

Author: bep

.gemini/config.yaml

@@ -2,7 +2,7 @@ have_fun: false
 memory_config:
   disabled: false
 code_review:
-  disable: false
+  disable: true
   comment_severity_threshold: HIGH
   max_review_comments: -1
   pull_request_opened: