feat(worker): PlanningSessionManager.DiscardAsync

src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -53,6 +53,19 @@ public sealed class PlanningSessionManager
         return new PlanningSessionStartContext(taskId, list.WorkingDir, files);
     }
 
+    public async Task DiscardAsync(string taskId, CancellationToken ct)
+    {
+        var ok = await _tasks.DiscardPlanningAsync(taskId, ct);
+        var sessionDir = Path.Combine(_rootDirectory, taskId);
+        if (Directory.Exists(sessionDir))
+        {
+            try { Directory.Delete(sessionDir, recursive: true); }
+            catch { /* best effort */ }
+        }
+        if (!ok)
+            throw new InvalidOperationException($"Task {taskId} was not in Planning state; nothing to discard.");
+    }
+
     public async Task<PlanningSessionResumeContext> ResumeAsync(string taskId, CancellationToken ct)
     {
         var task = await _tasks.GetByIdAsync(taskId, ct)