How to structure multi-project rules files to avoid prompt bloat

We have a monorepo with eleven packages. A single rules file grew to 900 lines and the agent started ignoring half of it.

What worked for us:

1. One short root rule (under 60 lines): language, test command, "never edit generated files".

2. Per-package rules that only apply to files under that path.

3. Rules as checklists, not essays. "Run pnpm test --filter pkg" beats a paragraph about testing philosophy.

---
globs: packages/billing/**
---
- Money is always integer cents.
- Every new endpoint needs an idempotency key.
- Run: pnpm --filter billing test

Since splitting, the agent follows the package rules almost every time. How do you structure yours?