docs(vercel): document Node.js runtime source of truth

[ZxlDragonDoctor]

Problem

Issue #187 reports Vercel Node.js version override warnings caused by multiple sources of truth. #192 already aligned engines, CI, Docker, and docs to Node.js 24.18.0, but the repository still lacked an explicit operator-facing statement of which source is authoritative and how to update the major in one pass.

Solution

Document package.json#engines.node as the Vercel source of truth in docs/vercel-deployment.md, list every location that must stay on the same major, and link that section from docs/dependency-maintenance.md.

Issue alignment

Addresses #187 after the Node 24 adoption in #192: remaining gap is the documented single update path and Project Settings alignment note. No maintainer won't-do on the issue (0 comments).

Core value: 消除 Vercel Node 版本多源歧义，给出可重复的升级路径。