Machine-readable description of the sandbox surface

Public, no auth - the probe an agent can run to VERIFY that a sandbox exists rather than take prose for it. Derives its facts from the live fail-closed sandbox engine instead of restating them by hand, so it cannot drift from the behaviour it describes. Sandbox is a property of the TOKEN, not a separate host: the same endpoints, the same bearer scheme, with WRITE verbs returning a simulated receipt and reads serving real data.