Restrict CI trigger to tags only

Remove gitea-actions branch trigger now that the runner setup is stable.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

.github/workflows/build.yml

@@ -3,8 +3,6 @@ on:
   push:
     tags:
       - '*'
-    branches:
-      - gitea-actions
 jobs:
   run-tests:
     runs-on: ubuntu-latest