feat(worker): queue interactive messages by default, interrupt opt-in

StreamingClaudeSession now buffers a mid-turn user message in a FIFO queue and
flushes one when the turn's result arrives (no implicit interrupt). InterruptAsync
only writes the control_request (no-op when idle); the resulting turn-end then
flushes any queued message. New InteractiveSessionService.InterruptAsync +
WorkerHub.InterruptInteractiveSession + IWorkerClient.InterruptInteractiveSessionAsync.

src/ClaudeDo.Worker/Planning/InteractiveSessionService.cs

@@ -125,6 +125,12 @@ public sealed class InteractiveSessionService
         await session.SendUserMessageAsync(text, ct);
     }
 
+    public async Task InterruptAsync(string taskId, CancellationToken ct)
+    {
+        if (_registry.TryGet(taskId, out var session))
+            await session.InterruptAsync(ct);
+    }
+
     public async Task StopAsync(string taskId, CancellationToken ct)
     {
         // StopAsync removes from registry and kills the session.