fix: split preview workflow into ci/pr for fork PR compatibility

Move PR comment posting to a workflow_run-triggered pr-* workflow
following the established two-workflow split pattern (see ci-size-data
→ pr-report). Uses post-pr-report-comment action with a unique HTML
marker for idempotent comment upserts with full pagination.

Addresses review feedback:
https://github.com/Comfy-Org/ComfyUI_frontend/pull/11289#discussion_r3090292990
https://github.com/Comfy-Org/ComfyUI_frontend/pull/11289#discussion_r3090292996

.github/workflows/ci-vercel-website-preview.yaml

@@ -25,7 +25,6 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: read
-      pull-requests: write
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6
@@ -45,31 +44,17 @@ jobs:
           URL=$(vercel deploy --prebuilt --token=${{ secrets.VERCEL_WEBSITE_TOKEN }})
           echo "url=$URL" >> "$GITHUB_OUTPUT"
 
-      - name: Comment preview URL on PR
-        uses: actions/github-script@v8
+      - name: Save PR metadata
+        run: |
+          mkdir -p temp/vercel-preview
+          echo "${{ steps.deploy.outputs.url }}" > temp/vercel-preview/url.txt
+          echo "${{ github.event.number }}" > temp/vercel-preview/number.txt
+
+      - name: Upload preview metadata
+        uses: actions/upload-artifact@v6
         with:
-          script: |
-            const body = `**Website Preview:** ${process.env.PREVIEW_URL}`
-            const { data: comments } = await github.rest.issues.listComments({
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              issue_number: context.issue.number,
-            })
-            const existing = comments.find(
-              (c) => c.user?.type === 'Bot' && c.body?.includes('**Website Preview:**')
-            )
-            const params = {
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              body,
-            }
-            if (existing) {
-              await github.rest.issues.updateComment({ ...params, comment_id: existing.id })
-            } else {
-              await github.rest.issues.createComment({ ...params, issue_number: context.issue.number })
-            }
-        env:
-          PREVIEW_URL: ${{ steps.deploy.outputs.url }}
+          name: vercel-preview
+          path: temp/vercel-preview
 
   deploy-production:
     if: github.event_name == 'push'

.github/workflows/pr-vercel-website-preview.yaml

@@ -0,0 +1,48 @@
+---
+name: 'PR: Vercel Website Preview'
+
+on:
+  workflow_run:
+    workflows: ['CI: Vercel Website Preview']
+    types:
+      - completed
+
+permissions:
+  contents: read
+  pull-requests: write
+  actions: read
+
+jobs:
+  comment:
+    runs-on: ubuntu-latest
+    if: >
+      github.repository == 'Comfy-Org/ComfyUI_frontend' &&
+      github.event.workflow_run.event == 'pull_request' &&
+      github.event.workflow_run.conclusion == 'success'
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Download preview metadata
+        uses: dawidd6/action-download-artifact@0bd50d53a6d7fb5cb921e607957e9cc12b4ce392 # v12
+        with:
+          name: vercel-preview
+          run_id: ${{ github.event.workflow_run.id }}
+          path: temp/vercel-preview
+
+      - name: Read preview metadata
+        id: meta
+        run: |
+          echo "url=$(cat temp/vercel-preview/url.txt)" >> "$GITHUB_OUTPUT"
+          echo "pr-number=$(cat temp/vercel-preview/number.txt)" >> "$GITHUB_OUTPUT"
+
+      - name: Write report
+        run: |
+          echo "**Website Preview:** ${{ steps.meta.outputs.url }}" > preview-report.md
+
+      - name: Post PR comment
+        uses: ./.github/actions/post-pr-report-comment
+        with:
+          pr-number: ${{ steps.meta.outputs.pr-number }}
+          report-file: ./preview-report.md
+          comment-marker: '<!-- VERCEL_WEBSITE_PREVIEW -->'
+          token: ${{ secrets.GITHUB_TOKEN }}