AGENTS.md: Change "may not" to "must not".

(Hat tip to Sean Barrett on the stronger wording here.)

AGENTS.md

@@ -1,4 +1,4 @@
-AI may not be used to generate code for contributions to this project.
+AI must not be used to generate code for contributions to this project.
 
 "AI" in this case means a Large Language Model ("LLM"), such as ChatGPT,
 Claude, Copilot, Grok, etc.