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/fleet/src/Fleet.Core/VarInt.cs

@@ -51,16 +51,4 @@ public static class VarInt
 
         return false;
     }
-
-    public static bool TryReadUInt32LittleEndian(ReadOnlySpan<byte> source, out uint value)
-    {
-        if (source.Length < 4)
-        {
-            value = 0;
-            return false;
-        }
-
-        value = BinaryPrimitives.ReadUInt32LittleEndian(source);
-        return true;
-    }
 }

backend/fleet/src/Fleet.Rpc/FleetMethods.cs

@@ -40,12 +40,6 @@ public static class FleetMethodTable
     ]);
 }
 
-/// <summary>二进制通道：0–15 保留给框架，产品从 16 开始。</summary>
-public static class FleetChannels
-{
-    public const byte FirstProductChannel = 16;
-}
-
 public sealed class ChallengeParams
 {
     [JsonPropertyName("nonce")]