(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.

Start by reading ROADMAP.md for the PR risk classification work and review the linked pre-human automated review gate feature. Trace where risk metadata, notifications, and Cedar approval rules are represented, then use the acceptance criteria as the definition of done: persisted tiers, tier-aware notifications, documented examples, and classifier-to-routing tests.

Written by the indexing model from the issue text.