feat(ext): add systemInstruction to ExecuteConfig

Expose a serializable `systemInstruction` string field on the
page-facing ExecuteConfig, mapped to `instructions.system` when
creating MultiPageAgent. Functions cannot cross the postMessage
boundary, so this flat string field replaces the object form.

Closes #359

packages/extension/src/entrypoints/main-world.ts

@@ -7,6 +7,12 @@ export interface ExecuteConfig {
 	model: string
 	apiKey?: string
 
+	/**
+	 * Global system-level instructions for the agent.
+	 * Equivalent to `AgentConfig.instructions.system`.
+	 */
+	systemInstruction?: string
+
 	/**
 	 * Whether to include the initial tab (that holds this main world script) in the task.
 	 * @default true
@@ -89,6 +95,7 @@ export default defineUnlistedScript(() => {
 						baseURL: config.baseURL,
 						model: config.model,
 						apiKey: config.apiKey,
+						systemInstruction: config.systemInstruction,
 						includeInitialTab: config.includeInitialTab,
 						experimentalIncludeAllTabs: config.experimentalIncludeAllTabs,
 					},