Amend what a thread says

Supersedes by EXACT disclosure id. Paraphrase supersedes nothing: two tellings of byte-identical content to the same audience are two tellings, forever.

There is deliberately NO audience field — an amend inherits its target's, so cross-audience amendment is unrepresentable rather than merely forbidden. Correcting the record with a different audience is a new disclosure to that audience.

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).