Claim the account's handle, or move to another one (C1b).

There is deliberately no "is this free?" endpoint: it would be a perfect enumeration oracle for other people's handles. The answer comes from this call, which sits behind the session and the rate limit.

A handle the account gives up is remembered. It returns to the pool only if nothing was ever published under it — otherwise a stranger could take it and revive its dead URLs with their own diagrams. Its former owner may always take it back.

Cookie-only (C2): moving the handle kills every named URL under the old one, which is not something a token in an agent's config should be able to do.