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

docs/dependency-maintenance.md

@@ -10,6 +10,8 @@ opens reviewable pull requests; it must not auto-merge dependency changes.
 - Review major updates quarterly and migrate them separately.
 - Coordinate Node.js changes across `package.json`, CI, Docker, Vercel, and
   contributor documentation instead of allowing a single-file runtime bump.
+  `package.json#engines.node` is the source of truth; see
+  [Vercel Node.js runtime source of truth](./vercel-deployment.md#nodejs-runtime-source-of-truth).
 
 ## Update boundaries
 

docs/vercel-deployment.md

@@ -14,7 +14,7 @@ Connect `FullAgent/fulling` to a Vercel project and keep these settings:
 - Install command: Vercel default (`npm install` from `package-lock.json`)
 - Build command: `npm run build`
 - Output directory: Vercel default
-- Node.js: 24.x, enforced by `package.json`
+- Node.js: 24.x (Project Settings), matching `package.json#engines.node`
 - Production branch: `main`
 
 No `vercel.json` file or application environment variables are required for this
@@ -24,6 +24,35 @@ are handled by the application itself.
 `output: 'standalone'` remains enabled because the Docker image consumes it.
 Vercel uses its native Next.js output.
 
+## Node.js runtime source of truth
+
+`package.json#engines.node` is the repository-owned source of truth for the
+Node.js major used by Vercel builds and Functions. Vercel documents that
+`engines.node` overrides the project dashboard selection, so a dashboard value
+that disagrees with the repository produces a version override warning even when
+the deployment succeeds.
+
+Keep these locations on the same major (currently **24**):
+
+| Location | Role |
+| -------- | ---- |
+| `package.json#engines.node` | Authoritative for Vercel and local installs |
+| `package-lock.json` root `engines` | Must match `package.json` after `npm install` |
+| `.github/workflows/ci.yml` `NODE_VERSION` | CI and release verification pin |
+| `Dockerfile` `FROM node:<version>-alpine` | Container build/runtime image |
+| Vercel Project Settings → Node.js Version | Dashboard mirror of the same major |
+| This document | Operator guidance |
+
+When changing the Node.js major, update every row in one pull request and verify
+a clean Preview deploy (and Docker build) before merge. Do not add a separate
+`.nvmrc` or `.node-version` unless the team deliberately changes this policy.
+
+To silence an override warning without a code change: set Vercel Project
+Settings → Node.js Version to the same major as `package.json` (currently `24.x`).
+A dashboard default that still points at a different major (for example Node.js
+22 or 24 while the repository declares another) will keep warning until both
+sides match.
+
 ## Zero-configuration behavior
 
 Without the legacy Auth and database variables: