[taskboard] clear stale dispatch errors on retry

lib/tasks.ts

@@ -303,6 +303,9 @@ export async function updateTask(id: number, input: Partial<TaskRecord>) {
     return null;
   }
 
+  const hasField = <K extends keyof TaskRecord>(field: K) =>
+    Object.prototype.hasOwnProperty.call(input, field);
+
   const mergedTags = Array.isArray(input.tags) ? input.tags.filter((tag) => typeof tag === "string") : existing.tags;
   const nextStatus = input.status ?? existing.status;
   const nextDispatchState = deriveDispatchState({ ...existing, ...input, tags: mergedTags }, existing);
@@ -334,9 +337,9 @@ export async function updateTask(id: number, input: Partial<TaskRecord>) {
       input.priority ?? existing.priority,
       nextStatus,
       JSON.stringify(mergedTags),
-      input.last_dispatch_at ?? existing.last_dispatch_at,
+      hasField("last_dispatch_at") ? input.last_dispatch_at ?? null : existing.last_dispatch_at,
       acknowledgedAt,
-      input.last_error ?? existing.last_error,
+      hasField("last_error") ? input.last_error ?? null : existing.last_error,
       completedAt,
       id,
     ],