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/website/src/pages/docs/features/chrome-extension/page.tsx

@@ -199,6 +199,7 @@ interface ExecuteConfig {
 	model: string     // Model name
 	apiKey?: string   // LLM AK
 
+	systemInstruction?: string // Global system-level instructions
 	includeInitialTab?: boolean
 	experimentalIncludeAllTabs?: boolean // Control all unpinned tabs in the window
 	onStatusChange?: (status: AgentStatus) => void