The transfer endpoint looked the caller's token up in the `token` table, so
an external-JWT login, which has no row there, could never move a session.
It now resolves the same token against the target workspace, which refuses a
token bound to the source whatever its kind, and still refuses job tokens.
Also stop calling the move target a fork in the model note, the menu item,
the editor placeholders and the moving notices: it can be the root or a
sibling.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>