feat: Add workflow_dispatch trigger to publish-upm workflow (#3)

Enables manual triggering of package publishing workflow from GitHub Actions UI.

This allows administrators to manually trigger package publishes without
waiting for automatic triggers.

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

Co-authored-by: Claude <[email protected]>

Author: tuha263

.github/workflows/publish-upm.yml

@@ -1,6 +1,8 @@
 name: Publish to UPM Registry
 
 on:
+  workflow_dispatch:  # Manual trigger
+  
   push:
     branches:
       - master