feat(enrichment): Cross-file caller-impact / dead-symbol analyzer

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Context

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

Detects: A changed/removed/renamed export with live callers in unchanged files the PR didn't update (a hidden compile/runtime break), and conversely new exports never referenced anywhere (dead-on-arrival).

Data source: Parse added/removed top-level declarations from the diff; resolve usages via GitHub Code Search API (search/code, repo-scoped) + contents API at headSha. Token-scoped, free within rate budget.

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