Cancel at period end

Requires confirmCancellation:true and explicit product. Cancels renewal at period end; Enterprise remains sales-managed. Reuse requestId for the same request. 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.