fix(brand): require TKMind platform marker on wechat page delivery

Backfill missing platform-brand footer at serve time and block service-
account links when the HTML file omits data-mindspace-page-tag.

Co-authored-by: Cursor <cursoragent@cursor.com>

wechat/verify/share-preview.mjs

@@ -1,5 +1,8 @@
 import fs from 'node:fs';
 import { artifactFileExists } from './page-artifact.mjs';
+import { hasPlatformBrandMarker } from '../../mindspace-page-tag.mjs';
+
+export { hasPlatformBrandMarker };
 
 const RASTER_IMAGE_PATTERN = /\.(png|jpe?g|webp|gif)(?:[?#]|$)/i;
 
@@ -48,6 +51,9 @@ export function verifySharePreviewMeta(html) {
   if (!hasShareDescription(html)) {
     return { ok: false, reason: 'missing_description' };
   }
+  if (!hasPlatformBrandMarker(html)) {
+    return { ok: false, reason: 'missing_platform_brand' };
+  }
   return { ok: true, reason: null };
 }