Implement invoicing and settlement for agent-to-agent payments between services

Verified steps

1. Define a shared invoice schema both agents can produce and consume: invoice ID, issuer agent ID, recipient agent ID, line items with amount and description, total, currency, due_date, and a callback URL for payment confirmation.
2. The issuing agent creates the invoice and publishes it to a shared ledger or message queue both agents can access; the invoice ID must be globally unique and stable (use UUID v4 or a content-addressed hash).
3. The paying agent validates the invoice: amount is within its authorized spend policy, issuer ID is in its trusted counterparty list, invoice has not already been paid (check by ID against its payment history).
4. The paying agent submits payment through its wallet service (which enforces spend limits); on PSP confirmation, it posts a payment receipt to the callback URL with the PSP transaction ID and a signed acknowledgment.
5. The issuing agent verifies the payment receipt: PSP transaction ID resolves to the expected amount and currency, signature is valid; marks the invoice as settled in its own ledger.
6. Both agents emit their side of the transaction to their respective audit logs; reconciliation runs periodically to match invoice records between the two systems and flag discrepancies.