Contract for budget gating and accounting around tool calls.

check/2 is an optional pre-spend gate (returns :ok to proceed or {:error, reason} to deny before executing); record/2 accounts for actual usage after the call. scope identifies the budget owner (e.g. %{agent: name, model: model}); est/usage are the planned/actual cost carriers. The default impl NoOp preserves current (untracked) behavior.