ci: restrict push trigger to main branch

Replace the wildcard `['**']` in the push trigger with `['main']`. This prevents redundant pipeline runs on non-main branches during push events.

.gitea/workflows/ci.yml

@@ -2,7 +2,7 @@ name: CI
 
 on:
   push:
-    branches: ['**']
+    branches: ['main']
   pull_request:
 
 jobs: