stacker: add PR template support and adopt gitstack body format
On PR creation, load `.github/PULL_REQUEST_TEMPLATE.md` from the worktree
and inject the first commit's body into the template's first `## ` section
(mirrors universe gitstack `inject_body_into_template`). Update path stays
body-less so user edits to the template-filled body survive re-push
(universe PR #1859413 regression guard).
Reformat the managed block to ReviewStack's gitstack layout: emit
`## 🥞 Stacked PR` header + `---------` separator inside the existing
`<!-- stacker:begin/end -->` markers. The HTML markers remain our
authoritative parsing boundary; the inner header + separator are what
ReviewStack's gitstackParser keys off so PRs are recognized as stacked.