chore(worker): external MCP tools return explicit results instead of empty responses

set_task_config/set_list_config now echo which fields were set vs cleared, get_list_config/get_task_config return an explicit found=false instead of null, and delete_list/run_task_now/reset_failed_task/remove_task_attachment return a confirmation record — matching the found/ok convention already used by batch_get_tasks and get_task_log.

tests/ClaudeDo.Worker.Tests/External/ListMcpToolsTests.cs

@@ -59,8 +59,10 @@ public sealed class ListMcpToolsTests : IDisposable
     {
         var created = await _sut.CreateList("gone", null, null, CancellationToken.None);
 
-        await _sut.DeleteList(created.Id, CancellationToken.None);
+        var result = await _sut.DeleteList(created.Id, CancellationToken.None);
 
+        Assert.True(result.Deleted);
+        Assert.Equal(created.Id, result.Id);
         Assert.Null(await _lists.GetByIdAsync(created.Id));
     }
 }