feat: sandbox plugins in null-origin iframes with postMessage RPC

hooks/use-plugin-slot-offers.ts

@@ -0,0 +1,17 @@
+'use client';
+
+// Reactive accessor for sandboxed plugin slot offers. Components that gate
+// layout on the *presence* of a plugin-supplied slot (e.g. a detail
+// sidebar) read from here instead of the legacy `usePluginStore.slots` map.
+
+import { useSyncExternalStore } from 'react';
+import type { SlotName } from '@/lib/plugin-types';
+import { offersForSlot, subscribe } from '@/lib/plugin-sandbox/registry';
+
+export function usePluginSlotOffers(slotName: SlotName) {
+  return useSyncExternalStore(
+    subscribe,
+    () => offersForSlot(slotName),
+    () => [],
+  );
+}