ci: trigger on main branch (renamed from trunk)

The Gitea repo's default branch is main; the local development branch
stayed trunk and pushes via `trunk:main` refspec. Workflow needs to
match what the remote sees.

run-name now interpolates ${{ gitea.ref_name }} so it accurately names
the branch on any future renames.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-push.yaml

@@ -1,9 +1,9 @@
 name: cpanel-importer Build and Push
-run-name: ${{ gitea.actor }} pushed a change to trunk
+run-name: ${{ gitea.actor }} pushed a change to ${{ gitea.ref_name }}
 on:
   push:
     branches:
-      - trunk
+      - main
     tags:
       - '20[0-9][0-9].[0-9][0-9].[0-9]+'