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/AttachmentMcpToolsTests.cs

@@ -134,8 +134,11 @@ public sealed class AttachmentMcpToolsTests : IDisposable
         var filePath = Path.Combine(_storeRoot, task.Id, "remove.txt");
         Assert.True(File.Exists(filePath));
 
-        await sut.RemoveTaskAttachment(task.Id, "remove.txt");
+        var result = await sut.RemoveTaskAttachment(task.Id, "remove.txt");
 
+        Assert.True(result.Removed);
+        Assert.Equal(task.Id, result.TaskId);
+        Assert.Equal("remove.txt", result.FileName);
         Assert.False(File.Exists(filePath));
         await using var vCtx = _db.CreateContext();
         var row = await new TaskAttachmentRepository(vCtx).GetAsync(task.Id, "remove.txt");