fix(graduate): align Dockerfile + workflow to Node 22 (PR #13 codex P2)

PR #13 codex review (P2): the new `engines.node: ">={{NODE_VERSION}}"`
in package.json.standard.template renders as ">=22", but the graduated
deployment artifacts (Dockerfile multi-stage + GitHub Actions workflow
in graduate.sh.template) still pinned Node 20. In environments with
`engine-strict=true` (common org default), graduated projects' npm ci
or pnpm install would fail purely due to the Node 20 < 22 mismatch.

- Dockerfile FROM lines 53/58/64: node:20-alpine → node:22-alpine
- Deploy workflow line 134: setup-node node-version '20' → '22'

Both numbers are now consistent with the engine floor and with
package.json.standard.template's {{NODE_VERSION}} = 22 placeholder
default.

Refs: PR #13 review by chatgpt-codex-connector (P2)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sgwannabe

plugins/preview-forge/assets/graduate.sh.template

@@ -50,18 +50,18 @@ fi
 if [[ ! -f Dockerfile ]]; then
   cat > Dockerfile <<'DOCKERFILE'
 # Preview Forge — generated on graduate to pro/max
-FROM node:20-alpine AS deps
+FROM node:22-alpine AS deps
 WORKDIR /app
 COPY package*.json ./
 RUN npm ci --omit=dev
 
-FROM node:20-alpine AS builder
+FROM node:22-alpine AS builder
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 RUN npm run build
 
-FROM node:20-alpine
+FROM node:22-alpine
 WORKDIR /app
 ENV NODE_ENV=production
 COPY --from=builder /app/.next ./.next
@@ -131,7 +131,7 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: actions/setup-node@v6
-        with: { node-version: '20' }
+        with: { node-version: '22' }
       - run: npm ci
       - run: npm run build
       - run: docker build -t $IMAGE .