feat(ci): run on pull requests

Only on branches from forks, though - to avoid duplicate workflow
runs, as workflows on branches in this GitHub repository are already triggered
by the `push` clause

.github/workflows/ci.yaml

@@ -2,9 +2,11 @@ name: ci
 
 on:
   push:
+  pull_request:
 
 jobs:
   build:
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
     runs-on: ubuntu-latest
     permissions:
       contents: write