The Boston Celtics have a lot of work to do heading into the summer. The quick, disappointing end to Boston’s playoff run has put an added emphasis on upgrading the supporting cast , but a few things have to change ahead of time.  “We have to use what we can to build this thing moving forward and add to it. Whatever moves that means, I have no idea,” Brad Stevens said about future moves last week. “But I also, I don't take for granted how good we've been when we've been full. When we've been full, and all on the court and playing together, we've been a good basketball team. Those are hard to get, so we just have to be better around the margins.” So despite the recent media storm surrounding Jaylen Brown , it seems pretty clear that he’s not going anywhere.…