Add a URL or pasted text to the desk's knowledge, then propose an update.

202, not 200: folding new material in is a compile, and a compile is model calls. The route validates synchronously — the owned-host check especially, because "that host isn't yours" must land while the human is still looking at the field — and hands the rest to propose_desk_knowledge_update. The result arrives as a PROPOSED version on this page; nothing activates itself.