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);
+}