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/Lifecycle/TaskMergeService.cs

@@ -70,6 +70,10 @@ public sealed class TaskMergeService
     public const string StatusMergedVerifyPending = "merged_verify_pending";
     public const string StatusVerifyPassed = "verify_passed";
     public const string StatusUntrackedCollision = "untracked_collision";
+    // Nothing landed -- the target working dir has uncommitted changes. Its own status rather
+    // than a Blocked reason string so callers can offer the autostash retry (MergeAsync's
+    // autostash: true) without matching on prose.
+    public const string StatusDirtyTarget = "dirty_target";
 
     public const string StatusReverted       = "reverted";
     public const string StatusConflictAborted = "conflict_aborted";
@@ -375,7 +379,8 @@ public sealed class TaskMergeService
         bool leaveConflictsInTree,
         CancellationToken ct,
         IProgress<ProgressNotificationValue>? progress = null,
-        bool skipVerify = false)
+        bool skipVerify = false,
+        bool autostash = false)
     {
         var (task, list, wt, verifyCommand) = await LoadMergeContextAsync(taskId, ct);
 
@@ -400,8 +405,10 @@ public sealed class TaskMergeService
                 return Blocked("working directory is not a git repository");
             if (await _git.IsMidMergeAsync(list.WorkingDir, ct))
                 return Blocked("target working directory is mid-merge");
-            if (await _git.HasChangesAsync(list.WorkingDir, includeUntracked: false, ct))
-                return Blocked("target working tree has uncommitted changes");
+            // autostash hands the dirty tree to git itself (merge --autostash) instead of refusing.
+            if (!autostash && await _git.HasChangesAsync(list.WorkingDir, includeUntracked: false, ct))
+                return new MergeResult(StatusDirtyTarget, Array.Empty<string>(),
+                    "target working tree has uncommitted changes");
 
             var currentBranch = await _git.GetCurrentBranchAsync(list.WorkingDir, ct);
             if (!string.Equals(currentBranch, targetBranch, StringComparison.Ordinal))
@@ -417,7 +424,7 @@ public sealed class TaskMergeService
             var message = string.IsNullOrWhiteSpace(commitMessage)
                 ? DefaultMergeMessage(task, list)
                 : commitMessage;
-            var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, message, ct);
+            var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, message, ct, autostash);
             if (exitCode != 0)
             {
                 List<string> files;
@@ -973,13 +980,15 @@ public sealed class TaskMergeService
         }
     }
 
-    public Task<MergeResult> ApproveAndMergeAsync(string taskId, string targetBranch, CancellationToken ct)
-        => ApproveAndMergeAsync(taskId, targetBranch, leaveConflictsInTree: false, ct);
+    public Task<MergeResult> ApproveAndMergeAsync(
+        string taskId, string targetBranch, CancellationToken ct, bool autostash = false)
+        => ApproveAndMergeAsync(taskId, targetBranch, leaveConflictsInTree: false, ct, autostash: autostash);
 
     public async Task<MergeResult> ApproveAndMergeAsync(
         string taskId, string targetBranch, bool leaveConflictsInTree, CancellationToken ct,
         IProgress<ProgressNotificationValue>? progress = null,
-        bool skipVerify = false)
+        bool skipVerify = false,
+        bool autostash = false)
     {
         var (task, list, wt, verifyCommand) = await LoadMergeContextAsync(taskId, ct);
 
@@ -1070,7 +1079,7 @@ public sealed class TaskMergeService
         // MergeAsync transitions the task WaitingForReview -> Done on a successful merge.
         // Remove the worktree on approve (matching the unit-merge path) so merged
         // worktrees don't pile up; the merge commit on the target branch is the record.
-        return await MergeAsync(taskId, target, removeWorktree: true, commitMessage: "", leaveConflictsInTree, ct, progress, skipVerify);
+        return await MergeAsync(taskId, target, removeWorktree: true, commitMessage: "", leaveConflictsInTree, ct, progress, skipVerify, autostash);
     }
 
     /// <summary>