Function requestSecret

Ask the user to bind one of their stored secrets to this app (SECRETS_SPEC §5, the powerbox flow — modeled on requestSpace()). The host draws a picker of only the user's matching secrets; the user picks, declines, or creates one. On success the host records a durable (appKey, secretId) grant and resolves with a SecretGrant (handle + metadata, never the value).

No existence oracle (T20/T27): a decline, an ungranted secret, and a nonexistent secret are indistinguishable — all reject with a SecretError cancelled; the app never sees the list it chose from.