Add private-preview branch to CI workflow triggers

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: mbroshi-stripe

.github/workflows/main.yml

@@ -14,6 +14,7 @@ on:
     branches:
       - master
       - beta
+      - private-preview
       - sdk-release/**
       - feature/**