Add cross-repo change-impact convention

AGENTS.md

@@ -90,6 +90,17 @@ Endpoint shapes + auth model are documented canonically in `../recap-relay/AGENT
 - **Endpoints called** — `/relay/{transcribe, transcribe-url, jobs/:id, summarize-url, summarize-url/:jobId/events, analyze, tts, balance, capabilities, policy, credits/packages, credits/buy, credits/invoice/:id, user-tier, user-tier/:id, tier-invoice, tier-zaprite-order, tier-plans, expiring-subscriptions}`. Settle webhooks land on the relay side, never here.
 - **Files** — `server/providers/relay.js` (transcribe/summarize/analyze/tts + balance + tier reads/writes), `server/relay-capabilities.js` (capabilities poll), `server/billing-routes.js` (tier purchase orchestration), `server/credits-purchase.js` (credit-pack purchase proxy), `server/subscription-reminders.js` (polls `/relay/expiring-subscriptions`). (`/relay/policy` is a small inline proxy in `index.js`.)
 
+### Cross-repo changes (sibling: `../recap-relay`)
+
+This repo and the relay (`../recap-relay`) share a live client/server contract — the
+`/relay/*` endpoints, the `X-Recap-*` headers, request/response shapes, and tier/credit
+semantics. **Before finishing any change that touches that boundary, check whether
+`../recap-relay` needs a matching change.** If you add/rename/remove a relay call, alter a
+payload shape or header, or shift tier/credit/billing behavior, update the relay side too —
+and reflect it in BOTH repos' `AGENTS.md` (the contract docs) and `ROADMAP.md` (if it's
+staged work). Purely local changes (UI, library handling, packaging) don't need this. When
+unsure whether a change is contract-affecting, assume it is and check.
+
 ## Always
 
 - **Bump the version before `make install`.** StartOS dedupes sideloads by version string — installing the same version twice silently no-ops. Use `make bump` or edit `startos/versions/index.ts` + add a `vN.ts` file. Applies to EVERY iteration, even a one-line edit.