docs(changelog): Add entry for build download command

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

Author: runningcode

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### New Features ✨
+
+- Add `sentry-cli build download` command to download installable builds (IPA/APK) by build ID ([#3221](https://github.com/getsentry/sentry-cli/pull/3221)).
+
 ## 3.3.3
 
 ### Internal Changes 🔧