Add a default gemini config.

This is based on https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#custom-configuration

Disabled the "have fun" and "summary" (as the PR summary should already
contain this)

Author: andy31415

.gemini/config.yaml

@@ -0,0 +1,9 @@
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: true