(orchestration): Tiered human review routing by PR risk

## Component

API or orchestration

## Describe the feature

Route PRs to **different human review requirements** based on **PR risk classification** (and optional code-area rules): auto-notify only senior reviewers for critical paths, skip human for docs-only low-risk, etc.

## Use case

Review is the new bottleneck. Teams experiment with **risk compartments** in the codebase—more scrutiny on critical paths, lighter review on internal/docs changes—and conserving human attention.

## Proposed solution

1. Extend **PR risk classification** (roadmap) output to `review_tier: minimal | standard | elevated`.
2. Blueprint maps tiers to: required reviewers (GitHub teams), Cedar approval rules, notification templates.
3. Optional: auto-merge when tier=minimal and all automated gates pass (Cedar-gated).

## Other information

- Builds on roadmap **PR risk classification**; this draft adds **routing** UX not spelled out in ROADMAP.md.
- Pairs with **feature-pre-human-automated-review-gate**.

## Acknowledgements

- [ ] I may be able to implement this feature
- [ ] This might be a breaking change

## Acceptance criteria

- [ ] Risk tier persisted on task/PR metadata.
- [ ] Notification plane respects tier (who gets heads-up).
- [ ] Documented examples for docs-only vs. auth-critical paths.
- [ ] Tests for classifier → routing mapping.

まず、PR リスク分類の作業について ROADMAP.md を読み、リンクされている人手によるレビュー前の自動レビューゲート機能を確認してください。リスクメタデータ、通知、Cedar の承認ルールがどこで表現されているかを追跡し、そのうえで受け入れ基準を完了の定義として使用してください：永続化されたティア、ティアを考慮した通知、文書化された例、classifier から routing までのテスト。

索引モデルが issue の本文から書いたものです。