Shared Renovate preset for every Bennett app

The rules were copied into each app's renovate.json, so they were identical only by luck
and a change meant six edits — the Testcontainers guard existed in two repos and not the
other four. Apps now extend this.

Also records the two majors that are blocked upstream rather than by our code:
typescript-eslint peers at typescript <6.1, eslint-plugin-react peers at eslint <=9.7.

README.md

@@ -0,0 +1,23 @@
+# renovate-config
+
+One Renovate preset, shared by every app on the [Bennett platform](../platform).
+
+Apps carry a three-line `renovate.json`:
+
+```json
+{ "extends": ["local>austin/renovate-config"] }
+```
+
+Rules used to be copied into each repo, which meant they were identical only by luck — and a change was
+six edits. The Testcontainers guard, for instance, existed in two repos and not the other four.
+
+## What it says
+
+- **Platform releases automerge.** They are our own code, tested and scanned before publish, and the
+  app's own build still gates the image push. Third-party majors do not automerge.
+- **Testcontainers majors are off.** 2.0 renamed the module artifacts, so the POM will not even parse.
+- **TypeScript and ESLint majors are off**, and this is temporary. They are blocked upstream:
+  `typescript-eslint` peers at `typescript <6.1`, `eslint-plugin-react` peers at `eslint <=9.7`.
+  Re-enable when those ship support — the code itself is ready, it builds clean on TypeScript 7.
+
+Changing a rule here changes it everywhere on Renovate's next run.