ci: add merge_group trigger to all required-check workflows

[Blackgaurd]

adds merge_group trigger to both CI workflows so GitHub's merge queue can run checks on temporary queue branches (gh-readonly-queue/...). without this, queued PRs stall indefinitely with no error.

path filters are intentionally omitted from the merge_group trigger — queue branches won't match path patterns, so omitting them ensures CI always runs for queued PRs.

[github-actions]

• main
  • ci: add merge_group trigger to all required-check workflows 👈
    • ci: add git-town stack visualization action #336
      • docs: add branch protection setup script and stacking workflow guide #337

[Blackgaurd]

closing — merge queue not available on free tier for public repos, removing merge_group CI changes