Withdraw one scope link

Bottom on the link, and an APPEND: the original Scope record stays where it was and a ScopeBottom lands beside it, so who took this memory out of the phase and when stays answerable. The memory reverts to the unscoped default unless other live links remain — and unscoped means it applies EVERYWHERE, not nowhere.

Requires the coord:write scope. Accepts all three lanes: a session JWT (bounded by the caller's role on this workspace) or an il_* machine credential whose own scope set, workspace set and validity window cover it — both ceilings apply and neither can raise the other, so a credential carrying coord:write is still refused where the role does not permit it.

A credential with no scope record resolves to LEGACY_DEFAULT, which does NOT include coord:write — mint a key with it explicitly (POST /v2/workspaces/{id}/keys).