ci: run CI on PRs targeting any branch

Supports stacked PRs — previously CI only ran on PRs to main.
Push triggers remain main-only.

Author: mfazekas

.github/workflows/ci.yml

@@ -4,8 +4,6 @@ on:
     branches:
       - main
   pull_request:
-    branches:
-      - main
   merge_group:
     types:
       - checks_requested