feat(ui): dirty Ziel-Verzeichnis beim Oeffnen des Merge-Dialogs melden statt nach dem Fehlschlag

MergeTargetsDto/MergeTargets bekommen TargetDirty (via HasChangesAsync,
includeUntracked: false). Das Merge-Modal zeigt Warnung + Stash-Retry
sofort in InitializeAsync statt erst nach dem dirty_target-Fehlschlag;
der REVIEW-Block in der Detail-Pane zeigt denselben Hinweis neben
Approve. Der dirty_target-Rueckgabepfad bleibt als Absicherung bestehen.

Behebt Befund F6 aus docs/test-session-2026-09-16.md.

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -20,7 +20,8 @@ public sealed record MergeResult(
 public sealed record MergeTargets(
     string DefaultBranch,
     IReadOnlyList<string> LocalBranches,
-    string DefaultCommitMessage);
+    string DefaultCommitMessage,
+    bool TargetDirty = false);
 
 // VerifyExitCode/VerifyDurationMs/VerifyOutputTail are only populated when the caller asked for
 // a verify run (runVerify=true) AND the list has a verify command configured AND the preview came
@@ -875,7 +876,8 @@ public sealed class TaskMergeService
 
         var current  = await _git.GetCurrentBranchAsync(list.WorkingDir, ct);
         var branches = await _git.ListLocalBranchesAsync(list.WorkingDir, ct);
-        return new MergeTargets(current, branches, defaultMessage);
+        var dirty    = await _git.HasChangesAsync(list.WorkingDir, includeUntracked: false, ct);
+        return new MergeTargets(current, branches, defaultMessage, dirty);
     }
 
     /// The commit message a merge uses when the caller passes none. Built here rather than in the