Update release notes for v3.3.1 writing improvements

RELEASE-NOTES.md

@@ -5,6 +5,7 @@
 ### Improvements
 
 - Updated `brainstorming` skill to require autonomous recon before questioning, encourage recommendation-driven decisions, and prevent agents from delegating prioritization back to humans.
+- Applied writing clarity improvements to `brainstorming` skill following Strunk's "Elements of Style" principles (omitted needless words, converted negative to positive form, improved parallel construction).
 
 ### Bug Fixes