feat(health): add codebase health score

[bntvllnt]

Summary

• Add health / get_health_score with gateable score, per-file maintainability, CRAP, coverage source, risk hotspots, and actions.
• Add hotspots --metric risk using the shared risk scorer.
• Add CH-P2-05 chained health E2E, registry/MCP parity coverage, docs/llms/roadmap sync, and real-repo verifier coverage.

Validation

• pnpm lint
• pnpm typecheck
• pnpm build
• pnpm test (36 files, 475 tests)
• pnpm verify:cli-real (71 pass, 0 fail)
• code-review artifact: /tmp/code-review-health-score.json — CLEAN, 80/80 hunks, 10/10 rules

Notes

• Health reads only root-local coverage/coverage-final.json or coverage/coverage.json; no arbitrary coverage path input.
• Boundary enforcement stays in the next P2 slice.