docs: Add changelog entry for validate-pr action

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stephanie-anderson

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Features
+
+- Add validate-pr composite action for validating non-maintainer PRs against contribution guidelines and enforcing draft status ([#153](https://github.com/getsentry/github-workflows/pull/153))
+
 ## 3.3.0
 
 ### Features