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.

backend/tests/MoShiro.Backend.Tests/OpenApiConformanceTests.cs

@@ -53,7 +53,7 @@ public sealed partial class OpenApiConformanceTests
         foreach ((string path, _) in doc["paths"]!.AsObject())
         {
             string[] prefixes = ["/api/public/", "/api/admin/", "/api/agent/v1/"];
-            string[] roots = ["/healthz", "/metrics", "/install", "/install.ps1", "/install/manifest", "/install/manifest.sig", "/api/release/offer"];
+            string[] roots = ["/healthz", "/metrics", "/install", "/install.ps1", "/install/manifest", "/install/manifest.sig", "/install/compose/{file}", "/api/release/offer"];
             Assert.True(prefixes.Any(p => path.StartsWith(p, StringComparison.Ordinal)) || roots.Contains(path), $"{path} is outside the documented layers");
             foreach (string segment in path.Split('/', StringSplitOptions.RemoveEmptyEntries))
             {