feat: archive mvp demo trace acceptance

mvp/demo/payment-timeout-acceptance.md

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+# Payment Timeout Acceptance Case
+
+## Goal
+
+Validate that the MVP can diagnose a payment timeout incident and expose the complete trace for replay.
+
+## Input
+
+- Session id: `mvp-demo-payment-timeout-001`
+- Question: `支付接口最近出现超时，请结合知识库、日志和指标判断可能原因，并给出修复建议。`
+- Profile: `mvp-demo`
+
+## Acceptance Criteria
+
+1. Chat returns a successful answer with the same session id.
+2. Trace API returns session metadata, final answer, ordered agent steps, and ordered tool invocations.
+3. Trace contains enough evidence to explain which tools were used and whether verifier/self-evaluation was persisted.
+4. Feedback can be submitted for the same session id.
+5. A follow-up trace query shows the persisted feedback value.
+
+## Trace Fields To Inspect
+
+- `data.session.query`
+- `data.session.answer`
+- `data.session.selfEvaluation`
+- `data.session.feedback`
+- `data.steps[*].agentName`
+- `data.steps[*].thought`
+- `data.toolInvocations[*].toolName`
+- `data.toolInvocations[*].inputParams`
+- `data.toolInvocations[*].outputPreview`
+- `data.toolInvocations[*].retrievalDetails`
+- `data.summary`
+
+## Known Limits
+
+- This case is not a full offline test. It still requires valid infrastructure for chat, persistence, vector search, and model calls.
+- Mock logs and metrics are enabled by the `mvp-demo` profile to make those evidence tools repeatable.
+- Sensitive configuration cleanup is deferred by current MVP priority.