ci: serialize branch verification runs

Author: saagpatel

.github/workflows/ci.yml

@@ -3,12 +3,20 @@ name: CI
 on:
   push:
     branches: [main]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
   pull_request:
     branches: [main]
 
 jobs:
   test:
     runs-on: macos-latest
+    timeout-minutes: 20
     steps:
       - uses: actions/checkout@v6