Readiness probe for the staged voice-model upgrade executor (agents:read). Returns the model-upgrades-v1 capability contract: `available`, `mutations_enabled`, `notice_ready` and the remaining `prerequisites`. Deliberately not env-gated — every flag reports false until a trusted provider-receipt adapter is implemented, so a 200 here means "the contract is readable", never "upgrades are live". Poll this before attempting an upgrade rather than inferring readiness from a successful HTTP status.