release_notes: update RELEASE_NOTES.md

Trim the first release note bullet at the comma so it becomes a single concise sentence, per the one-sentence rule.

Author: sg-doc-holiday[bot]

RELEASE_NOTES.md

@@ -14,7 +14,7 @@ hide_toc: false
 
 ## 🚀
 
-- Extended AI chat history retention to 30 days, keeping past conversations available longer before cleanup.
+- Extended AI chat history retention to 30 days.
 - Made the Region property optional in the Build ARN AWS action to simplify and flexibilize workflow configuration.
 - Added a new "Policy & Governance" category in the Templates gallery to make related templates easier to discover.