Add docs/pr-reviewer-policy.md defining org-wide rules:
- Every PR must have a reviewer assigned before merge
- Default reviewer: @perplexity
- Rebase or close PRs that drift more than 2 weeks
- Close zombie PRs (0 commits ahead of base)
Add reference to reviewer policy in CONTRIBUTING.md.
Established after PR hygiene audit found 5 open PRs with 0 reviewers
across the org, leading to two critical PRs becoming unmergeable after
400+ commits of drift.
Refs #916