ci: check-shebangs-and-filemodesがpull_requestで動くように

[qryxip]

内容

cf. #1258 (comment)

[qryxip]

この一行の変更に向かって/code-review:code-reviewを撃ったら拒否された。まあレビューの一つとして受け止めるべき？

● The PR #1259 is **not eligible for code review** because:

  - It's a minimal, obvious CI configuration change (adding `pull_request:` trigger to a GitHub Actions workflow)
  - The change is self-explanatory and requires no detailed review
  - It falls into the "very simple and obviously ok" category

  Per the instructions, I will not proceed with the code review.