However, like many flaws with HN's underdeveloped software, you can't actually do that on HN because the parser includes the trailing > in the URL. So your best bet here is with whitespace.
I'd argue that the world has moved on since 1998. We live in a world full of URL parsers of various abilities, and doggedly enclosing URLs in angle brackets because of a memo written so long ago it also references gopher seems stubborn to no obvious gain. Given the state of the modern internet, I definitely wouldn't consider angle brackets to be the best way to delimit URLs.
However, like many flaws with HN's underdeveloped software, you can't actually do that on HN because the parser includes the trailing > in the URL. So your best bet here is with whitespace.