feat(merge): Stash-Option fuer uncommittete Aenderungen im Ziel-Tree

Ein dirty Ziel-Arbeitsverzeichnis gibt jetzt den eigenen Status
dirty_target zurueck statt blocked. Die UI bietet daraufhin einen
Retry an, der per git merge --autostash stasht, merged und die
Aenderungen danach wiederherstellt - opt-in pro Merge:

- Approve & Merge: Bestaetigungsdialog
- Merge-Modal: Button "Aenderungen stashen und mergen"
- MCP (merge_task/review_task) wirft weiterhin, kein autostash dort

src/ClaudeDo.Worker/Hub/WorkerHub.cs

@@ -478,7 +478,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
     }
 
     public Task<MergeResultDto> MergeTask(
-        string taskId, string targetBranch, bool removeWorktree, string commitMessage)
+        string taskId, string targetBranch, bool removeWorktree, string commitMessage, bool autostash = false)
         => HubGuard(async () =>
         {
             // A blank message is handed through deliberately — TaskMergeService builds the
@@ -488,7 +488,9 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
                 targetBranch ?? "",
                 removeWorktree,
                 commitMessage,
-                CancellationToken.None);
+                leaveConflictsInTree: false,
+                CancellationToken.None,
+                autostash: autostash);
             return new MergeResultDto(r.Status, r.ConflictFiles, r.ErrorMessage);
         });
 
@@ -683,7 +685,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         if (!result.Ok) throw new HubException(result.Reason ?? "dequeue failed");
     }
 
-    public Task<MergeResultDto> ApproveReview(string taskId, string targetBranch)
+    public Task<MergeResultDto> ApproveReview(string taskId, string targetBranch, bool autostash = false)
         => HubGuard(async () =>
         {
             bool hasChildren;
@@ -698,7 +700,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
                 return new MergeResultDto(unitResult.Status, Array.Empty<string>(), unitResult.Reason);
             }
 
-            var r = await _mergeService.ApproveAndMergeAsync(taskId, targetBranch ?? "", CancellationToken.None);
+            var r = await _mergeService.ApproveAndMergeAsync(taskId, targetBranch ?? "", CancellationToken.None, autostash);
             if (r.Status == TaskMergeService.StatusBlocked)
                 throw new HubException(r.ErrorMessage ?? "approve failed");
             return new MergeResultDto(r.Status, r.ConflictFiles, r.ErrorMessage);