feat: signed releases and a host-side updater for container nodes

CI signs the image digest of each agent and probe release; the panel only
stores and forwards it. Nodes write the manifest to their data directory and
a host updater verifies the signature with a local public key, pulls by
digest, restarts the unit and rolls back when it is unhealthy. The panel can
list releases, ask a node to update and show the reported update state.

docs/fleet/update.md

@@ -1,5 +1,7 @@
 # Fleet：通用更新（Fleet.Update / fleet-updater）
 
+> **实现状态**：容器节点（Agent、Probe）的更新已经实现，见 [deployment.md](../deployment.md) 的「容器节点的更新」：同样的信任模型（离线发布密钥签名、本机配置公钥和仓库、`seq` 防重放、面板只通知和请求），清单里是镜像 digest，更新器是宿主机上的 `monitor-update`。本文下面描述的纯二进制方案（`Fleet.Update`、`fleet-updater`、`fleet-release`、健康信标）还没有实现。
+
 ## 1. 原则
 
 - **与产品无关**：`fleet-updater` 通过本地配置声明它管理哪些应用，一台机器上可以同时管理多个产品。