One file covers 10%. Ten layers across three tiers make Copilot auditable.

Most teams turn on GitHub Copilot in agent mode, write a single .github/copilot-instructions.md, and stop. That one file covers roughly 10% of the configuration surface VS Code exposes for Copilot. This playbook maps the other 90%: ten configuration layers grouped into three memory tiers, hot, warm, and cold, so every rule loads when it is relevant, nothing gets truncated, and Copilot behavior becomes auditable, versioned, and owned instead of tribal.