fix(worker): start_list_handler ehrlich machen, wenn keine App verbunden ist

ConnectedClientTracker zaehlt verbundene UI-Clients (WorkerHub.OnConnectedAsync/
OnDisconnectedAsync). StartListHandler prueft ihn vor dem Broadcast und liefert
bei 0 verbundenen Clients Requested=false plus Reason statt unbedingtem true.
Tool-Beschreibung und external-mcp.md an das reale Verhalten angepasst.

src/ClaudeDo.Worker/Hub/ConnectedClientTracker.cs

@@ -0,0 +1,19 @@
+namespace ClaudeDo.Worker.Hub;
+
+/// <summary>
+/// Counts SignalR clients (the ClaudeDo App) currently connected to <see cref="WorkerHub"/>.
+/// A <c>Hub</c> instance is transient per connection, so this lives as a DI singleton and is
+/// updated from <see cref="WorkerHub.OnConnectedAsync"/>/<see cref="WorkerHub.OnDisconnectedAsync"/>
+/// — the only way an out-of-band caller (external MCP's start_list_handler, which only
+/// broadcasts) can tell whether anything is listening before claiming it did something.
+/// </summary>
+public sealed class ConnectedClientTracker
+{
+    private int _count;
+
+    public bool AnyConnected => Volatile.Read(ref _count) > 0;
+
+    public void Increment() => Interlocked.Increment(ref _count);
+
+    public void Decrement() => Interlocked.Decrement(ref _count);
+}

src/ClaudeDo.Worker/Hub/WorkerHub.cs

@@ -74,6 +74,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
     private readonly TicketImportService _ticketImport;
     private readonly HandlerRestartCoordinator? _handlerRestart;
     private readonly HandlerRoundState? _roundState;
+    private readonly ConnectedClientTracker? _clientTracker;
 
     public WorkerHub(
         QueueService queue,
@@ -114,7 +115,8 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         TicketClientFactory ticketClients = null!,
         TicketImportService ticketImport = null!,
         HandlerRestartCoordinator? handlerRestart = null,
-        HandlerRoundState? roundState = null)
+        HandlerRoundState? roundState = null,
+        ConnectedClientTracker? clientTracker = null)
     {
         _queue = queue;
         _waker = waker;
@@ -155,6 +157,21 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         _ticketImport = ticketImport;
         _handlerRestart = handlerRestart;
         _roundState = roundState;
+        _clientTracker = clientTracker;
+    }
+
+    // Tracks connected UI clients so an out-of-band caller (external MCP's start_list_handler)
+    // can tell whether the broadcast it just sent had anyone to receive it.
+    public override Task OnConnectedAsync()
+    {
+        _clientTracker?.Increment();
+        return base.OnConnectedAsync();
+    }
+
+    public override Task OnDisconnectedAsync(Exception? exception)
+    {
+        _clientTracker?.Decrement();
+        return base.OnDisconnectedAsync(exception);
     }
 
     // Persistence boundary for the session_skills JSON-array columns (task/list/global).