feat(enrichment): Changed-line coverage-delta

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Context

A REES (review-enrichment service) analyzer. Tier: solid.

Detects: Added/changed lines NOT covered by any test, computed from the project's OWN CI coverage report — real measured gaps on exactly the touched lines, not a guess about whether tests 'look' present.

Data source: GitHub Actions API: runs for headSha → artifacts → download lcov/coverage-final/cobertura, parse per-line hit counts, intersect with added-line numbers from patch hunks. Free with the short-lived token.

This is heavy/external/historical analysis the no-checkout headless claude --print reviewer cannot do; the REES returns it as a brief block the engine splices into the review (additive + fail-safe).

Implementation (established pattern, all inside review-enrichment/)

1. Finding type + BriefFindings key in src/types.ts
2. src/analyzers/<name>.ts — pure, inject fetch for tests
3. Register in src/brief.ts ANALYZERS registry
4. Render a public-safe block in src/render.ts
5. node:test units against dist/ + a live smoke against the real data source

Deliverables

• The analyzer + wiring + tests + a verifiable brief block (file:line or package@version)
• Clean PR off main (zero engine conflict; outside the engine tsc/vitest/codecov scope)

Parent: #1499