Payment guard middleware for AI agents. Use whenever any skill is about to initiate a payment. Runs a preflight check against the user's policy (payments enabled, per-transaction limit, monthly cap). Returns ALLOW, DENY, or CONFIRM_REQUIRED. Other payment-capable skills must call signet-preflight before proceeding and signet-record after a successful payment.

AI代理的支付防护中间件。在任何技能即将发起支付时调用。会根据用户的策略（是否启用支付、单笔限额、月度上限）进行预检，返回ALLOW、DENY或CONFIRM_REQUIRED。其他具备支付功能的技能必须在执行前调用signet-preflight，并在支付成功后调用signet-record。