Disable workflow on subrepo branches in BushelCloud

Only run the bushel-cloud-build workflow on PRs targeting main branch.
This prevents the workflow from failing on subrepo branches like 'mistkit'
where the local MistKit path dependency is not available.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: leogdion

.github/workflows/bushel-cloud-build.yml

@@ -9,8 +9,10 @@ on:
   # Allow manual trigger
   workflow_dispatch:
 
-  # Build on PRs for validation
+  # Build on PRs for validation (only PRs targeting main)
   pull_request:
+    branches:
+      - main
 
 # Prevent concurrent builds
 # Why cancel-in-progress?