Change GEMINI.md to AGENTS.md (#6969)

Per https://antigravity.google/changelog, supported in 1.20.5. Also note
https://agents.md

Assisted-by: Google Antigravity with Gemini

CONTRIBUTING.md

@@ -232,7 +232,7 @@ as well as helpful tooling that will ease the contribution process. For example,
 #### Using AI-based contribution tools
 
 If you are using an AI assistant to help you contribute, or if you are an AI
-assistant yourself, please consult [GEMINI.md](/GEMINI.md) for high-density
+assistant yourself, please consult [AGENTS.md](/AGENTS.md) for high-density
 technical context and tips.
 
 All submissions to Carbon need to follow our