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

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.