codexloop quota and capacity

QuotaExhausted is a billing wall with no resets_at — it is never
waitable and always needs a human to top up. Rate-limit windows are
waitable.

CapacityState variants

Available  # green light
ThrottleExhausted  # retry_after: timedelta | None
WindowExhausted  # resets_at: datetime | None, window: str
QuotaExhausted  # reason: str — TERMINAL, never waitable
AuthFailed  # reason: str — TERMINAL
TransientBackendError  # retry_after: timedelta | None

is_waitable(state) returns False only for QuotaExhausted and
AuthFailed.

Body-first classification

classify(signals) in domain/classify.py maps TurnSignals to a
CapacityState using this ladder:

1. Body wins. error.code / error.type outrank HTTP status.
2. Auth: error.code / error.type in ErrorClass.AUTH, or status 401
   → AuthFailed.
3. Quota: error.code / error.type in ErrorClass.QUOTA →
   QuotaExhausted (terminal).
4. Window: ErrorClass.WINDOW → WindowExhausted with resets_at from
   plan windows.
5. Throttle: ErrorClass.THROTTLE → ThrottleExhausted with
   retry_after.
6. Transient: ErrorClass.TRANSIENT or status 5xx →
   TransientBackendError.
7. Status 429 with no body marker → WindowExhausted(resets_at=None).
8. Fallback: Available.

High used_percent never outranks a billing marker. Completion claims
never outrank capacity rejection.