fix(mindspace): lower page access password minimum to 6 characters

Align publication validation, portal gate HTML, H5 publish UI, MCP docs,
and page-data skill guidance so passwords like 888888 are accepted while
3-character values such as 888 remain rejected.

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

server/portal-publication-routes.mjs

@@ -39,7 +39,7 @@ export function renderPublicationPasswordGate(
   <form method="post" action="${action}">
     <h1>此页面受密码保护</h1>
     <p>请输入发布者提供的访问密码。密码不会写入链接或浏览器日志。</p>
-    <input type="password" name="password" minlength="8" maxlength="128" required autocomplete="current-password">
+    <input type="password" name="password" minlength="6" maxlength="128" required autocomplete="current-password">
     <button type="submit">访问页面</button>
   </form>
 </body>