feat: install nodes and the backend from a static compose file; remove dead code [CI]

Agent, Probe and Backend now start from one static compose.yaml plus a local
.env: the panel (or the release assets, for the very first Backend) provides
the file, the panel's command writes the public values into .env, checks the
file's SHA-256 and runs compose up. Updating is compose pull && compose up -d.
The image is chosen by the CI channel (:latest, or MOSHIRO_VERSION in .env),
not by the panel; the previous one-line script stays as a fallback.

- packaging/compose/{agent,probe,backend}/compose.yaml are the single source,
  embedded in the Backend and served at /install/compose/{file}; the root
  compose.yaml moved there. Releases also carry them and list them in the
  signed SHA256SUMS.
- The container entrypoint defaults the node name to the host name.
- Join token creation returns a compose block (.env, url, hash, per-platform
  commands); the admin dialog leads with it.
- Remove code with no references (HotLayer members, LiveHub.SubscriberCount,
  QueryService.ParseParams, ReleaseService.Forget, a few constants) and reduce
  the install-code lookup to the role it is for.

docs/security/access.md

@@ -175,7 +175,12 @@ macOS 用 `shasum -a 256 -c`，Windows 用 `Get-FileHash` 比较，不通过就
 - 脚本向面板取已签名的清单（和更新用的是同一份，发布密钥签名、`seq` 防回滚），用**脚本里内置的发布公钥**验签，再按清单里的 digest 拉镜像（`repo@sha256:…`）。Backend 改不了 digest，最多让节点装到一个旧的合法版本。
 - 下载压缩包安装的人，用 Release 里签名的 `SHA256SUMS` 核对（见 [deployment.md](../ops/deployment.md)）。
 
-**③ 其余**
+**③ compose 安装（默认）**
+
+- 管理端的容器安装是：建目录，把参数写进本地 `.env`（面板地址、一次性码、创世指纹、属主，都是公开的值），下载面板提供的静态 compose 文件、核对 SHA-256，`compose up -d`。compose 文件是静态的（和安装脚本一样嵌在 Backend 里，也作为 Release 资产发布），镜像是 `:latest` 或 `.env` 里固定的版本，**镜像地址由 CI 渠道决定，不由面板决定**，面板被攻破也改不了；Backend 第一次部署时没有面板，就从 Release 资产下载。
+- 代价：客户端不再验发布签名，更新（`compose pull && compose up -d`）靠 TLS 加「注册表可信」这个假设；想要更严，在 `.env` 里把 `MOSHIRO_VERSION` 固定到 digest，或者用宿主机更新器（清单有发布签名，不健康自动回滚）。详见 [deployment.md](../ops/deployment.md)。
+
+**④ 其余**
 
 - 角色（Agent 还是 Probe）、镜像仓库地址仍由代码决定，但镜像身份由签名清单决定。
 - macOS 的 Developer ID 签名和公证、Windows 的 Authenticode 需要证书，目前没有做。