feat(import): add stable per-resource source keys to the importer contract (#614)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

packages/plugin-runtime/src/PluginInstance.ts

@@ -169,6 +169,7 @@ export class PluginInstance {
             type: "import_response",
             importer: this.#mod.importer.name,
             resources: reply.resources as ImportResources,
+            sourceKeys: reply.sourceKeys ?? null,
           };
           this.#sendPayload(context, replyPayload, replyId);
           return;