Read remaining quotas and plan entitlement for the authenticated company. Call this when you need to know whether an active plan is in effect before registering OIDC clients or creating login links. Creating login links, minting extra API keys, and registering end users all require an active plan (the bootstrap key does not). Requires a company API key (aa_...) or an MCP OAuth access token. Returns planId, planStatus, periodYm, included, used, remaining, and hardCap. It does not change billing or charge a card. To start a human Pro checkout, use billing_checkout. billing_machine_pay only returns prepaid REST instructions. To manage an existing Stripe subscription, use billing_portal.