fix(worker): prevent PlanningMergeOrchestrator double-drain race and orphaned state

src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs

@@ -56,11 +56,9 @@ public sealed class PlanningMergeOrchestrator
                 .Where(c => c.Worktree is not null && c.Worktree.State != WorktreeState.Merged)
                 .Select(c => c.Id));
 
-        _states[planningTaskId] = new State
+        var state = new State { TargetBranch = targetBranch, RemainingSubtaskIds = queue };
-        {
+        if (!_states.TryAdd(planningTaskId, state))
-            TargetBranch = targetBranch,
+            throw new InvalidOperationException($"Merge already in progress for {planningTaskId}.");
-            RemainingSubtaskIds = queue,
-        };
 
         await _broadcaster.PlanningMergeStarted(planningTaskId, targetBranch);
         await DrainAsync(planningTaskId, ct);
@@ -70,6 +68,9 @@ public sealed class PlanningMergeOrchestrator
     {
         if (!_states.TryGetValue(planningTaskId, out var state)) return;
 
+        var keepState = false;
+        try
+        {
             while (state.RemainingSubtaskIds.TryDequeue(out var subtaskId))
             {
                 state.CurrentSubtaskId = subtaskId;
@@ -84,6 +85,7 @@ public sealed class PlanningMergeOrchestrator
                 if (result.Status == TaskMergeService.StatusConflict)
                 {
                     await _broadcaster.PlanningMergeConflict(planningTaskId, subtaskId, result.ConflictFiles);
+                    keepState = true;
                     return;
                 }
 
@@ -92,6 +94,7 @@ public sealed class PlanningMergeOrchestrator
                     await _broadcaster.PlanningMergeConflict(
                         planningTaskId, subtaskId,
                         new[] { result.ErrorMessage ?? "merge blocked" });
+                    keepState = true;
                     return;
                 }
 
@@ -100,9 +103,13 @@ public sealed class PlanningMergeOrchestrator
 
             state.CurrentSubtaskId = null;
             await FinalizePlanningDoneAsync(planningTaskId, ct);
-        _states.TryRemove(planningTaskId, out _);
             await _broadcaster.PlanningCompleted(planningTaskId);
         }
+        finally
+        {
+            if (!keepState) _states.TryRemove(planningTaskId, out _);
+        }
+    }
 
     private async Task FinalizePlanningDoneAsync(string planningTaskId, CancellationToken ct)
     {