Scope an existing memory to a regime

The companion to remember's regimes: [...], for a memory that already exists — a classification applied after the fact.

Keep the returned scope. It is the ScopeLinkId and the only handle that can ever withdraw this link: descope names the LINK, not the (memory, regime) pair.

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).