fix(custody): key scoped main quota by the roster primary account

Under scoped custody, core's state fence keeps a main quota only when its
accountIdentity equals claustrum.primaryAccount.accountId. The OpenCode plugin
keyed main quota by the local mainAccountId slot, so every header harvest and
usage poll for main was dropped at save. A migrated seat, whose slot id differs
from the provider account id, therefore never persisted main quota: every
process polled /api/oauth/usage on its own and hit 429 backoff.

Resolve the main quota key to the scoped primary account id when scoped custody
is active; keep the local slot key otherwise.