ARCHITECTURE.md: the system as it stands, in the present tense. TRIGGER when: creating or editing `ARCHITECTURE.md`; a *why* paragraph lands in the README; adding a component, datastore, external dependency or invariant; user asks how the system fits together or where a trade-off goes. SKIP when: install/run instructions (`readme-conventions`), or dev setup, pre-commit or CI (`contributing-conventions`).
