workflows: close incomplete PRs

- Close PRs unless the body preserves 75% of the current template and
  includes human content or checked boxes.
- Use `pull_request_target` with no checkout and fetch only
  `.github/PULL_REQUEST_TEMPLATE.md` from `main` by API.
- Inline the template matching Ruby so the workflow has no helper script
  to fetch and each `gh api` call remains isolated to one step.
- Require the `PR_BODY` env binding to exist instead of silently
  defaulting it inside the shell.