fleet: require a minimum version of the peer at the handshake [CI]

MoShiro__MinAgentVersion makes the backend refuse agents below it at hello (fleet_version_required); fleet.min_server_version or MOSHIRO_MIN_SERVER_VERSION makes the agent drop a backend below it. Either side waits five seconds and shakes hands again, because the other one may be updating.

backend/agent/src/MoShiro.Agent/AgentApp.cs

@@ -353,6 +353,7 @@ public sealed class AgentApp : IDisposable, IAsyncDisposable
             ServerVerifier = _trust,
             NodeId = _identity.NodeId,
             NodeVersion = Version,
+            MinServerVersion = () => Environment.GetEnvironmentVariable("MOSHIRO_MIN_SERVER_VERSION") is { Length: > 0 } required ? required : _config.Fleet.MinServerVersion,
             Capabilities = [MoShiroCapabilities.Base, MoShiroCapabilities.KindsPrefix + string.Join(',', _listening ? BlockCatalog.AgentKinds : BlockCatalog.TargetKinds)],
             HelloMeta = HelloMeta,
             HeartbeatExtra = HeartbeatExtra,

backend/agent/src/MoShiro.Agent/AgentConfig.cs

@@ -40,6 +40,10 @@ public sealed class FleetSection
     /// <summary>节点自己要求有属主（passkey）：没有就不运行。见 <c>OwnerGuard</c>。</summary>
     [JsonPropertyName("require_owner")]
     public bool RequireOwner { get; set; } = true;
+
+    /// <summary>面板至少要是哪个版本才继续连接（如 <c>0.4.2-dev.80</c>）；空 = 不要求。不够就不发任何东西，5 秒后重新握手（面板可能正在更新）。环境变量 <c>MOSHIRO_MIN_SERVER_VERSION</c> 优先。</summary>
+    [JsonPropertyName("min_server_version")]
+    public string? MinServerVersion { get; set; }
 }
 
 public sealed class ListenSection