[Enhancement] Evaluation Client: extensible hook for external reasoning verification before payment settlement

Start by reviewing issue #393 and the Evaluation Client design discussion, then compare the requested pre-settlement callback, structured trace, and transaction metadata with the AgentCore Payments flow described here. Done would mean a decided interface and semantics for verification, including how a verdict can prevent settlement and appear in the audit record.

Following up on #393 (Evaluation Client — Lifecycle, Orchestration & Online Pipeline) — raising a specific design question before the interface solidifies.

Context

AgentCore Payments introduces a new failure mode that goes beyond what existing guardrails cover: a payment flow where the agent's reasoning was internally consistent but the decision to transact was poorly grounded. The AWS blog post acknowledged this directly under roadmap: "stronger buyer intent verification."

The current observability stack (logs, metrics, traces in the AgentCore console) captures what happened after the fact. What I'm not seeing is a hook for pre-settlement verification — a point in the execution loop where external logic can inspect the agent's reasoning trace and return a structured verdict before AgentCore finalizes the payment.

Concrete ask

When the Evaluation Client (#393) is designed, would it support:

1. A pre-settlement callback interface — e.g. on_before_payment(trace, context) -> VerificationResult — that can short-circuit the transaction if the reasoning doesn't meet a defined threshold?
2. A structured trace format that evaluation logic can consume deterministically (not just raw logs)?
3. A way to attach the verification result as metadata to the transaction record, so audit trails include both what was paid and why the reasoning was considered sound?

This pattern is especially relevant for regulated use cases (financial services, healthcare, high-stakes procurement) where "the agent decided to transact" is not sufficient — you need a provable record that the reasoning behind the decision was evaluated.

Happy to share a reference architecture sketch if it would help the design discussion — particularly around the trace-format and threshold-semantics questions.