# Trace UI workbench

## Why

The MVP already persists diagnosis trace data and exposes it through
`GET /api/diagnosis/{sessionId}/trace`, but reviewers still need to inspect raw
JSON to answer basic audit questions:

- Which agents ran, in what order, and with what persisted counts?
- Which evidence tools executed, how many times, and did they succeed?
- Which facts did the Verifier check, and which evidence refs support them?
- Did Planner only select skill metadata while Executor handled skill loading?
- What RAG retrieval details were used by `lookup_knowledge`?

This slows down demo review and makes trace quality issues harder to spot.

## What changes

- Add a static Trace workbench page served by Spring Boot static resources.
- Load a diagnosis trace by session id through the existing read-only Trace API.
- Render session summary, agent timeline, evidence tool ledger, verifier facts,
  skill boundary checks, and RAG retrieval details.
- Add a navigation entry from the existing chat page to the Trace workbench.

## Non-goals

- No new backend endpoint.
- No mutation of diagnosis sessions, agent steps, tool invocations, or feedback.
- No new frontend framework or build pipeline.
- No change to the persisted trace schema.

## Impact

- Frontend-only runtime surface under `src/main/resources/static`.
- Uses the existing `Result<T>` API response contract.
- Works with existing trace records, including sessions that lack verifier or RAG
  detail fields.