Configure automatic credit purchases

Enabling authorizes future purchases using the default payment method. Requires current catalogVersion, valid unit quantity and explicit monthlyPurchaseLimit as number of purchases. Disabling preserves other provider billing rules. requestId is the idempotency UUID. Requires billing.write, a verified person and owner role. Subject to rollout-agent-access-writes (default off) and allow-public-agent-writes before account/provider effects. Return and person handoff URLs always use the canonical Dashboard origin; redirect URLs, payment details and provider tokens are not accepted.