Add .gemini to gitignore

Change-Id: If30d355de800f0db979df508c76d300cf218de8f
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1202699
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>

.gitignore

diff --git a/.gitignore b/.gitignore
index 913ca23..d616fa9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,6 @@
 *.swp
 bazel-*
 .vscode
+
+# Coding Agents put workspace state here - we don't want to check that in.
+.gemini