Run checks on main

This will (1) make sure that anything we push directly passes CI, and also (2) cache the pnpm store on the main branch, which will make it available to PR runs as well and hopefully speed up CI a bit (see https://stackoverflow.com/a/75250061).``

.github/workflows/ci.yaml

@@ -3,6 +3,8 @@ name: CI checks
 on:
   pull_request:
     branches: [main]
+  push:
+    branches: [main]
 
 jobs:
   run-checks: