fix(wechat): guard bare page-continuation follow-ups from session loss and memory drift

Prevent chat-to-page flows like writing a poem then saying "做成页面吧" from opening
a fresh session without carried content, failing with a vague error, or letting Memory
page-template preferences replace the immediate topic.

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

wechat/prompts/page-generate.mjs

@@ -14,6 +14,7 @@ export function buildPageGenerateAgentPrompt(
     wantsDocx = false,
     imagePolicy = null,
     preferImmediateContext = false,
+    carriedSessionContent = '',
   } = {},
 ) {
   const topic = String(intent?.agentText ?? intent?.displayText ?? '').trim();
@@ -41,6 +42,13 @@ export function buildPageGenerateAgentPrompt(
         '',
       ].join('\n')
     : '';
+  const carriedSessionBlock = String(carriedSessionContent ?? '').trim()
+    ? [
+        '【待排版正文（来自上一轮会话，必须以此为主题，禁止改用 Memory 或其他任务）】',
+        String(carriedSessionContent).trim(),
+        '',
+      ].join('\n')
+    : '';
   const coverExample = imageBlock
     ? '<本轮 generate_image 返回的 asset.htmlSrc>'
     : 'assets/hero.jpg';
@@ -62,6 +70,7 @@ export function buildPageGenerateAgentPrompt(
     docxBlock,
     imageBlock,
     immediateContextBlock,
+    carriedSessionBlock,
     pageDataBlock,
     '步骤（必须全部完成）：',
     pageDataTask