Document Relay @required and export rules

Capture two conventions surfaced while building the top bar: use the
Relay @required directive to make expected-present nullable fields
non-null for consistent typing, and reserve default exports for the
component that lazy() imports as a bundle entry while everything else
uses named exports.

Signed-off-by: Émile Ré <emile@probo.com>

.cursor/rules/relay-required-directive.mdc

@@ -0,0 +1,48 @@
+---
+description: Use Relay @required to make expected-present nullable fields non-null
+globs: "**/*.tsx"
+alwaysApply: false
+---
+
+# Use `@required` for expected-present fields
+
+GraphQL schemas mark many fields nullable defensively, but at a given call site
+you usually **expect** a value. When a field is nullable in the schema but the
+component cannot meaningfully render without it, annotate it with `@required` so
+the **generated type is non-null**. This keeps typing consistent: callers stop
+threading `?.` / `?? ""` / `!` through code that always expects data, and a
+genuinely-missing value becomes a real signal instead of a silently-empty UI.
+
+Choose the action by what should happen when the value is actually absent:
+
+- `@required(action: THROW)` — the value is an invariant for this view (a page's
+  root entity, the `currentTrustCenter` a portal is built around). A null throws
+  on read and propagates to the nearest error boundary. Field becomes non-null.
+- `@required(action: LOG)` — a missing value should degrade gracefully: the null
+  bubbles to the nearest `@required` ancestor (or nulls the fragment data) and
+  Relay logs it. Use when the surrounding UI can render a fallback.
+- `@required(action: NONE)` — bubble nullability without logging; rarely needed.
+
+```graphql
+# GOOD — the view is built around this entity; THROW makes it non-null
+currentTrustCenter @required(action: THROW) {
+  organization {
+    name                    # already String! — no @required needed
+    logo { downloadUrl }    # legitimately optional — leave nullable
+  }
+}
+```
+
+```tsx
+// GOOD — non-null typing falls out of @required; no defensive chaining
+const { organization } = data.currentTrustCenter;
+const logoUrl = organization.logo?.downloadUrl ?? undefined; // logo stays optional
+```
+
+Do NOT use `@required` to silence nullability on fields that are *genuinely*
+optional (avatar, logo, optional description) — those keep their nullable type
+and get a real empty/fallback state. Do NOT use the `THROW` as control flow for
+an expected-empty case (it is an error path, not a branch). Do NOT annotate
+fields the schema already declares non-null (`String!`, `Organization!`).
+
+See `contrib/claude/relay.md` (Fragments → Required fields).