Only trigger CI push runs on master to avoid duplicate builds

Pushing to a branch with an open PR fired both the push and
pull_request triggers for the same commit. pull_request already
covers feature-branch commits, so push only needs to cover master.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -2,6 +2,7 @@ name: CI
 
 on:
   push:
+    branches: [master]
   pull_request:
 
 jobs: