feat(square): 新增发布广场功能，智能体/技能按广场分类展示

- 后端新增 PublishSquare/PublishSquaresSettings 配置与读写接口
- agents/skills 增加 square_id 字段及对应迁移脚本
- 前端新增发布广场设置页、技能转移选择器，画廊按广场分 Tab 展示
- lead_agent prompt 及若干持久化层适配调整

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/components/workspace/settings/appearance-settings-page.tsx

@@ -31,6 +31,7 @@ import {
 import { cn } from "@/lib/utils";
 
 import PrivateSquareSettingsPage from "@/pages/PrivateSquareSettingsPage";
+import PublishSquaresSettingsPage from "@/pages/PublishSquaresSettingsPage";
 import PromptPrefixSettingsPage from "@/pages/PromptPrefixSettingsPage";
 import RecommendedQuestionsPage from "@/pages/RecommendedQuestionsPage";
 
@@ -349,6 +350,8 @@ export function AppearanceSettingsPage() {
           <PromptPrefixSettingsPage embedded />
           <Separator />
           <PrivateSquareSettingsPage embedded />
+          <Separator />
+          <PublishSquaresSettingsPage embedded />
         </>
       )}
     </div>