ci: run on pull_request regardless of base branch (#11)

Drop the branches: [main] filter so stacked PRs whose base is another
branch in the same repo (e.g. PR #7 stacked on this one) still get
build-and-test runs. Keep the push trigger narrow to main so CI does
not fire on every force-push across feature branches.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MoonBoi9001

.github/workflows/ci.yaml

@@ -4,7 +4,6 @@ on:
   push:
     branches: [main]
   pull_request:
-    branches: [main]
 
 jobs:
   build-and-test: