Move the Node floor off EOL 20 to 24

CONTRIBUTING.md already states "Node.js 24 or above" and .nvmrc pins
v24.2.0, but package.json still declared `engines.node: ">=20"` and three
workflow entries still provisioned Node 20. Node 20 reached end of life on
2026-04-30, so the release-cutting job in particular was running on an
unsupported runtime.

Align the stragglers with the requirement the repo already documents:

- package.json (and the lockfile root it mirrors): >=20 -> >=24
- cut-release.yml: the `final` job's setup-node 20 -> 24
- sep-lifecycle.yml / sep-lifecycle-manual.yml: NODE_VERSION 20 -> 24

main.yml, markdown-format.yml, and render-seps.yml were already on 24 and
are unchanged. No dependency versions move; the lockfile diff is the root
`engines` field only.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>