Set up an e2e test suite

E2e tests verify the whole running system through the app (browser/API), not one module. They are the per-PR gate. Pairs with dev-local-setup (a reproducible local stack) and verifier-setup (which scaffolds the repo's /verify skill — the verify→ship loop this gate feeds into).

Where it lives

• Unit/integration tests stay inside each app/package — they own one module.
• System e2e is a dedicated top-level package (e.g. e2e/) — it spans all apps, so it belongs to none. Add it to the workspace if a monorepo.