Add config.yaml for Gemini Code Assist (#919)

The [Gemini Code
Assist](https://developers.google.com/gemini-code-assist/docs/overview)
app may have some value as a PR review aid, so I added it to this repo
as an experiment. However, its default behavior includes writing a
summary of the PR, and this is often pretty useless for us and just adds
noise. This config file tells the bot to do only PR reviews, and even
then, only flag high-priority problems.

Author: mhucka

.gemini/config.yaml

@@ -0,0 +1,28 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Summary: configure Gemini Code Assist (https://codeassist.google/).
+# See https://github.com/marketplace/gemini-code-assist for more info.
+
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: HIGH
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: true
+    include_drafts: false
+ignore_patterns: []