CostTrackingMiddleware

Zero the cost channel before a nested agent run.

Async variant of before_agent.

Charge every model request completed since the previous checkpoint.

Charge model requests that completed after the last model step.

Work outside the agent loop can spend once after_model has run for the final step: ReliableRubricMiddleware.aafter_agent runs a whole grading agent, and after_agent hooks run in reverse stack order, so this one drains after it. Anything that still spends later is charged on the next turn's first step rather than lost, but draining here keeps the turn's own checkpoint complete. A nested instance also transfers its cumulative local delta after the subagent completes.