feat(core): make execute_javascript honor AbortSignal

Expose the task AbortSignal as `signal` in the script scope so cooperative
code can cancel promptly, and re-check signal.throwIfAborted() after the
script settles to discard stale results.

Closes #537.

packages/core/src/tools/index.ts

@@ -181,12 +181,15 @@ tools.set(
 	'execute_javascript',
 	tool({
 		description:
-			'Execute JavaScript code on the current page. Supports async/await syntax. Use with caution!',
+			'Execute JavaScript code on the current page. Supports async/await syntax. Use with caution! ' +
+			'An `AbortSignal` named `signal` is available in scope: long-running async code MUST honor it ' +
+			'(e.g. `await fetch(url, { signal })`, or `signal.throwIfAborted()` in loops)',
 		inputSchema: z.object({
 			script: z.string(),
 		}),
-		execute: async function (this: PageAgentCore, input) {
+		execute: async function (this: PageAgentCore, input, { signal }) {
-			const result = await this.pageController.executeJavascript(input.script)
+			const result = await this.pageController.executeJavascript(input.script, signal)
+			signal.throwIfAborted()
 			return result.message
 		},
 	})

packages/extension/src/agent/RemotePageController.ts

@@ -133,8 +133,10 @@ export class RemotePageController {
 		return this.remoteCallDomAction('scroll_horizontally', args)
 	}
 
-	async executeJavascript(...args: any[]): Promise<DomActionReturn> {
+	async executeJavascript(script: string, _signal?: AbortSignal): Promise<DomActionReturn> {
-		return this.remoteCallDomAction('execute_javascript', args)
+		// `AbortSignal` is not structured-cloneable across the messaging boundary.
+		// The remote script runs without it
+		return this.remoteCallDomAction('execute_javascript', [script])
 	}
 
 	/** @note Managed by content script via storage polling. */

packages/page-controller/src/PageController.ts

@@ -376,13 +376,15 @@ export class PageController extends EventTarget {
 	}
 
 	/**
-	 * Execute arbitrary JavaScript on the page
+	 * Execute arbitrary JavaScript on the page.
+	 * The optional `signal` is exposed to the script scope so cooperative code
+	 * can abort promptly when the task is stopped.
 	 */
-	async executeJavascript(script: string): Promise<ActionResult> {
+	async executeJavascript(script: string, signal?: AbortSignal): Promise<ActionResult> {
 		try {
-			// Wrap script in async function to support await
+			// Wrap script in async function to support await, exposing `signal`.
-			const asyncFunction = eval(`(async () => { ${script} })`)
+			const asyncFunction = eval(`(async (signal) => { ${script} })`)
-			const result = await asyncFunction()
+			const result = await asyncFunction(signal)
 			return {
 				success: true,
 				message: `✅ Executed JavaScript. Result: ${result}`,