Expand a held grant into the chat/agreement ids it covers

Reach map for a grant you hold: the readable chat ids (and, for an org grant, agreement ids) inside its scope, ids + labels only, never content. A grant is a fence, not a listing: discovery says "you hold org:acme", this says which chats/agreements that covers so you can read them via context read (via=chat:<id> / agreement:<id>). Org scope is capped with an honest truncation count. Holder-only.