feat(wechat): require share preview meta before page link delivery

Block service-account page links when HTML lacks mindspace-cover and
description so WeChat forwards always get TKMind branding and thumbnail.

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

wechat/handlers/page-generate.mjs

@@ -1,10 +1,12 @@
 import { buildPagePublishFailureText } from '../prompts/page-generate.mjs';
 import { selectSendableHtmlArtifacts, verifyPageArtifactContent } from '../verify/page-artifact.mjs';
+import { filterSharePreviewReadyArtifacts } from '../verify/share-preview.mjs';
 
 export function evaluatePageGenerateSendableArtifacts({ verifiedArtifacts = [], confirmedArtifacts = [] } = {}) {
   const sendable = selectSendableHtmlArtifacts({ verifiedArtifacts, confirmedArtifacts });
   const verified = sendable.filter((artifact) => verifyPageArtifactContent(artifact).ok);
-  return verified.length > 0 ? verified : sendable;
+  const candidates = verified.length > 0 ? verified : sendable;
+  return filterSharePreviewReadyArtifacts(candidates);
 }
 
 /**
@@ -42,6 +44,17 @@ export function resolvePageGenerateOutcome({
     };
   }
 
+  const previewCandidates = filterSharePreviewReadyArtifacts(
+    selectSendableHtmlArtifacts({ verifiedArtifacts, confirmedArtifacts }),
+  );
+  if (previewCandidates.length === 0 && confirmedArtifacts.length > 0) {
+    return {
+      action: 'fail',
+      failureText: buildPagePublishFailureText({ missingSharePreview: true }),
+      reason: 'missing_share_preview',
+    };
+  }
+
   if (!String(reply?.text ?? '').trim() && sendable.length === 0) {
     return {
       action: 'fail',