Introspect the calling key

Use first in any session to confirm auth works and learn the key's org, project pin, scopes, and contract version. Works with any valid key regardless of scopes. Also carries the account snapshot behind the CLI status card: plan (the org's plan tier, e.g. pro or pro_plus; null on the Free plan) and credits ({ remaining, monthlyIncluded }; null only when the balance could not be read). Both are optional, best-effort fields; a billing-read failure never fails this endpoint.