Expose live resource subscriptions via a hook #1325

Background

ResourcesScreen renders a subscriptions: InspectorResourceSubscription[] prop showing which resources the user has subscribed to and when each was last updated. InspectorClient tracks subscriptions internally and dispatches notifications/resources/updated notifications, but there's no hook that surfaces the live list to React.

#1244's App.tsx passes subscriptions={[]} because no such hook exists yet. The result: onSubscribeResource / onUnsubscribeResource callbacks fire correctly, but the UI never reflects the subscription state — buttons feel no-op from the user's perspective even though the server-side subscriptions are real.

Scope

• Add a state manager — core/mcp/state/resourceSubscriptionsState.ts — that subscribes to the InspectorClient's resource-subscription events and maintains an InspectorResourceSubscription[]. Updates the lastUpdated field when notifications/resources/updated arrives for a subscribed URI.
• Add a useResourceSubscriptions(client, state) hook in core/react/.
• Wire App.tsx to pass the real list through.

Acceptance criteria

• Subscribing to a resource adds an entry visible in the Resources screen.
• Unsubscribing removes the entry.
• notifications/resources/updated updates the lastUpdated timestamp of the matching subscription.

Related

• Wire App.tsx to v2 core/ hook layer; remove demo stub from InspectorView #1244 — has subscriptions={[]} with a comment pointing here.