chore: add GEMINI.md, have it load AGENTS.md (#3246)

Apparently, Gemini doesn't automatically process AGENTS.md files. This
can be worked
around by creating GEMINI.md and telling it to read the AGENTS.md file.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
diff --git a/GEMINI.md b/GEMINI.md
new file mode 100644
index 0000000..285e0f5
--- /dev/null
+++ b/GEMINI.md
@@ -0,0 +1 @@
+@./AGENTS.md