fix: Clarify breaking change nature of preview features.

Author: chriswblake

.github/steps/3b-copilot-agent-mode.md

@@ -11,7 +11,7 @@ But... good news. Anyone can access the Insiders version if they know the tricks
 
 > [!IMPORTANT]
 > This activity is optional and ungraded.
-> Since the Insiders version is updated daily, please expect to see differences.
+> Since the Insiders version is updated daily, please expect to see differences or have breaking changes.
 
 ### What is "Agent" Mode?