feat(agents): add end-to-end review-loop behavior eval for orientation-first Scenario D

Tracks WI-02 from the orientation-first review integration epic (#2090).

An end-to-end behavioral evaluation is needed to validate that the full Scenario D loop — orientation → dispatch board → walk-back → emission — functions correctly against a representative sample diff.

**Acceptance criteria:**
- A sample diff fixture is defined under `.copilot-tracking/` or a dedicated eval directory.
- The eval exercises each loop stage: orientation floor emits a narrative + dispatch board, walk-back resolves a finding, emission produces structured output.
- Eval results are documented or captured in a way that confirms expected behavior.
- Any behavioral discrepancies uncovered are logged as follow-on issues.

**References:**
- Parent epic: #2090
- Affected agents: `.github/agents/coding-standards/code-review.agent.md`, `code-review-walkback.agent.md`, `code-review-explainer.agent.md`
Related to #2090

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Read .github/agents/coding-standards/code-review.agent.md, code-review-walkback.agent.md, and code-review-explainer.agent.md, then review parent epic #2090 for the intended Scenario D flow. Define a representative sample diff and an evaluation path under .copilot-tracking/ or a dedicated eval directory. Done means orientation, dispatch, walk-back, and emission are exercised with expected results captured, and discrepancies are logged as follow-on issues.

Written by the indexing model from the issue text.