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/extension/src/agent/RemotePageController.ts

@@ -133,8 +133,10 @@ export class RemotePageController {
 		return this.remoteCallDomAction('scroll_horizontally', args)
 	}
 
-	async executeJavascript(...args: any[]): Promise<DomActionReturn> {
-		return this.remoteCallDomAction('execute_javascript', args)
+	async executeJavascript(script: string, _signal?: AbortSignal): Promise<DomActionReturn> {
+		// `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. */