Stop parallel agents from editing shared code

Your team shares a TypeScript monorepo. This morning, 3 engineers each started an agent on an unrelated feature. All 3 agents also "improved" code in packages/common: one renamed a helper, one changed a function's signature and one added a dependency.

Each PR works alone. Together they conflict and break each other's builds, and it is the third time this month.

What do you change so this stops happening, and what do you do about today's 3 PRs?