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/handlers/page-generate.mjs

@@ -1,6 +1,6 @@
 import { buildPagePublishFailureText } from '../prompts/page-generate.mjs';
 import { selectSendableHtmlArtifacts, verifyPageArtifactContent } from '../verify/page-artifact.mjs';
-import { filterSharePreviewReadyArtifacts } from '../verify/share-preview.mjs';
+import { filterSharePreviewReadyArtifacts, verifyArtifactSharePreview } from '../verify/share-preview.mjs';
 
 export function evaluatePageGenerateSendableArtifacts({ verifiedArtifacts = [], confirmedArtifacts = [] } = {}) {
   const sendable = selectSendableHtmlArtifacts({ verifiedArtifacts, confirmedArtifacts });
@@ -48,10 +48,16 @@ export function resolvePageGenerateOutcome({
     selectSendableHtmlArtifacts({ verifiedArtifacts, confirmedArtifacts }),
   );
   if (previewCandidates.length === 0 && confirmedArtifacts.length > 0) {
+    const previewIssue = verifyArtifactSharePreview(
+      selectSendableHtmlArtifacts({ verifiedArtifacts, confirmedArtifacts })[0] ?? confirmedArtifacts[0],
+    );
     return {
       action: 'fail',
-      failureText: buildPagePublishFailureText({ missingSharePreview: true }),
-      reason: 'missing_share_preview',
+      failureText: buildPagePublishFailureText({
+        missingPlatformBrand: previewIssue.reason === 'missing_platform_brand',
+        missingSharePreview: previewIssue.reason !== 'missing_platform_brand',
+      }),
+      reason: previewIssue.reason ?? 'missing_share_preview',
     };
   }