feat: 新增内置通用助手 / list built-in General Assistant (#216)

## 中文
新增通用助手的双语展示条目，主文件明确声明 system/repository，sidecar 使用
listing-only。市场展示内置状态，更新复用 Agent 仓库合并；客户端负责离线安装与默认手动多会话。


独立公开源仓库：[agent-general-assistant](https://github.com/desirecore/agent-general-assistant)，初始内容已通过
PR #1 合入 main。客户端实现：desirecore/desirecore#4814。

验证：i18n 与 catalog metadata 均 0 errors（105
条既有条目警告），翻译新鲜度与公开内容检查通过；客户端已从真实 main 生成种子，真实 bundle verification 通过。

## English
Add the bilingual General Assistant listing with explicit
system/repository policies and listing-only availability. The client
owns offline provisioning and manual multi-session defaults; updates use
the agent repository merge flow.

The public source repository is available on main. Client integration:
desirecore/desirecore#4814. i18n/catalog validation passed with 0 errors
and 105 existing warnings, alongside translation freshness and
publication checks. The real main-based seed and bundle verification
passed.

README.md

@@ -133,6 +133,8 @@ The raw pointer must pass the [exported agent client schema](schemas/market-agen
 
 An inline agent may include concise, text-only `USAGE.md` notes for prerequisites, authorization, and safety boundaries. Localized variants use `USAGE.<locale>.md`, resolved by requested locale → source locale → default locale → unsuffixed file. Usage is displayed separately from `fullDesc`, which also enters the agent's runtime context. Clients may truncate long notes; put extensive supporting material in skill `references/` instead. Relative images do not render in the usage section.
 
+System-managed listings such as [General Assistant](agents/general-assistant/agent.json) explicitly declare the `system/repository` policy pair in the primary file and use `listing-only` availability in the sidecar. The client ships the runtime content offline and merges updates from the Agent’s own repository; the market does not install or uninstall another copy.
+
 Examples: [inline DesireCore agent](agents/desirecore/agent.json) and [DingTalk Workspace pointer](agents/dingtalk-workspace/entry.json).
 
 ### Teams