feat(agents): 内置智能体筛选(仅管理员)+ 内置 agent 默认用途标签

- 后端 scope=builtin(user_id IS NULL)加入 sql/memory 的 scope 过滤;
  /api/agents 扩展 scope 正则并加 admin 闸门(非管理员请求 403)
- 新增 _builtin_agent_tag_seed:启动时幂等给功能型内置 agent 按用途打标签
  (roundtable-* → 多智能体规划,ai-writing-* → AI写作),只追加不覆盖;
  无用途的内置 agent 不打标签
- 前端 agent-gallery 新增「内置智能体」Tab(仅 system_role==admin 可见),
  scope 类型在 api.ts/hooks.ts 同步

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

offline-backend-20260512/backend/app/gateway/app.py

@@ -980,6 +980,18 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
                 # re-run the heavy sync even though we just did it.
                 _services_mark_sync_fresh()
                 logger.info("Legacy folder-backed agents synced at startup")
+                # 内置功能型 agent(roundtable-* / ai-writing-*)按用途默认打标签,
+                # 供管理员「内置智能体」筛选按用途归类。幂等、只追加、失败不阻塞启动。
+                try:
+                    from app.gateway.routers._builtin_agent_tag_seed import (
+                        ensure_builtin_agent_purpose_tags,
+                    )
+
+                    tag_store = getattr(app.state, "tag_store", None)
+                    if tag_store is not None:
+                        await ensure_builtin_agent_purpose_tags(agent_store, tag_store)
+                except Exception:
+                    logger.exception("Built-in agent purpose-tag seeding failed at startup (non-fatal)")
         except Exception:
             logger.exception("Legacy agent sync failed at startup (non-fatal)")