Document chris-testing replacement runbook

[shiny-code-bot]

Summary

• document that chris-testing now carries runner hygiene responsibilities beyond a basic runner role
• add a replacement/cutover sequence for provisioning, warm-builder verification, baseline evidence, hygiene dry-run evidence, label/variable migration, and rollback
• keep replacement guidance aligned with Launchplane-owned hygiene and runner-control boundaries

Validation

• git diff --check
• markdownlint via editor validation reported 0 errors for docs/runner-host-hygiene.md