Runbook — Create or load an operational runbook

When invoked with a topic (e.g., /runbook ARI management group):

1. Search runbooks/ for existing runbooks matching the topic
2. If found, load and display the runbook
3. If not found, create a new one using this template:

# Runbook: {{title}}

## Purpose
[One sentence — when and why to use this]

## Prerequisites
-

## Steps
1.