(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.

Beginne mit der Lektüre von ROADMAP.md zur Arbeit an der PR-Risikoklassifizierung und prüfe die verknüpfte Funktion für ein automatisiertes Review-Gate vor der menschlichen Prüfung. Verfolge, wo Risikometadaten, Benachrichtigungen und Cedar-Genehmigungsregeln dargestellt werden, und verwende dann die Akzeptanzkriterien als Definition of Done: persistierte Stufen, stufenabhängige Benachrichtigungen, dokumentierte Beispiele und Tests vom Classifier bis zum Routing.

Vom Indexierungsmodell aus dem Issue-Text verfasst.