## Summary
- Add explicit `sourceMetadata.externalReviewCheckout` review routing
with fail-closed validation.
- Log the selected review checkout so operator/runtime tasks can verify
where review ran.
- Cover metadata extraction, invalid metadata fallback,
`fn_review_step`, and workflow `stepReview` routing.
## Test Plan
- `cd packages/engine && corepack pnpm exec vitest run
src/__tests__/review-checkout.test.ts
src/__tests__/reviewer-workspace.test.ts`
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Review routing now supports explicit external checkout overrides via
review checkout metadata, using the resolved external checkout as the
review working directory when valid.
* Logging now highlights the selected review routing target and warns
when external checkout metadata is present but invalid.
* **Bug Fixes**
* Missing/blank/invalid/relative/nonexistent/non-git paths now fail
closed to the task worktree instead of falling back.
* Metadata priority is enforced strictly (custom fields → branch context
→ source metadata → root), preventing lower-priority fallback when
higher-priority data is invalid.
* **Tests**
* Added/expanded automated coverage for override and fail-closed
behavior, including workspace-mode routing semantics.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
a03aae7f66