## Why

The evaluation baseline is now complete, but developers still need a repeatable way to decide whether a new Agent run regressed against that baseline. A deterministic baseline diff turns saved reports into an actionable regression signal instead of a static artifact.

## What Changes

- Add a baseline diff model that compares two `DiagnosisEvalReport` objects.
- Detect aggregate changes such as pass-rate drops, verdict distribution shifts, tool-call cost changes, and duration changes.
- Detect per-case changes such as pass/fail regression, verdict changes, keyword coverage changes, evidence coverage loss, and missing/new cases.
- Add JSON and Markdown diff output suitable for review.
- Document how to interpret the diff in the eval docs.

## Capabilities

### New Capabilities

- None.

### Modified Capabilities

- `diagnosis-eval-harness`: Extend the existing evaluation harness so a current report can be compared against the saved baseline report.

## Impact

- Affects eval-only Java code under `src/main/java/com/superbiz/agent/eval`.
- Adds focused tests under `src/test/java/com/superbiz/agent/eval`.
- Updates `mvp/eval` documentation and may add sample diff output.
- No production Agent runtime, API, database schema, or external dependency changes are expected.