Add a .geminiignore file (#1224)

Tell Gemini CLI what it should or should not ignore. Takes precedence
over `.gitignore` (which Gemini CLI also reads). This PR adds only one
pattern for now: a pattern that Gemini CLI does not seem to handle
correctly based on `.gitignore` alone.

Author: mhucka

.geminiignore

@@ -0,0 +1,22 @@
+# Copyright 2026 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: tell Gemini CLI what it should or should not ignore. Takes precedence
+# over .gitignore (which Gemini CLI also reads), thus can be used to un-ignore
+# files that shouldn't be committed to git yet should be visible to Gemini. Also
+# used to ignore things usually in people's global .gitignore files.
+
+# The next one (for Emacs versioned backups) _should_ be covered by *~ in our
+# .gitignore, but Gemini currently doesn't seem to interpret *~ that way.
+*.~*~