persist-env-var-with-rebuild

Make a BUILD-TIME environment variable survive a production redeploy. Use whenever a feature needs a value baked at build — any NEXT_PUBLIC_* (inlined into the browser bundle), the language set, Stripe keys + product ids, a custom API/DB URL, a feature flag, analytics IDs. Write the value into the slot's own app/.env.local through the proper setter, then trigger a rebuild: the slot-scoped build bakes the slot's own .env.local. Use when adding payments, a new integration key, a feature flag, or any value the app reads at build time — and to avoid the silent "saved but the app never sees it" bug. Self-sufficient: works for a lone agent, no Hermes required.