fix: harden browser provider activation

browse/src/browser-manager.ts

@@ -382,6 +382,7 @@ export class BrowserManager {
     // BROWSE_EXTENSIONS_DIR points to an unpacked Chrome extension directory.
     // Extensions only work in headed mode, so we use an off-screen window.
     const extensionsDir = process.env.BROWSE_EXTENSIONS_DIR;
+    if (extensionsDir) assertHeadedBrowserProvider();
     const { STEALTH_LAUNCH_ARGS, buildGStackLaunchArgs } = await import('./stealth');
     const launchArgs: string[] = [...STEALTH_LAUNCH_ARGS, ...buildGStackLaunchArgs()];
     let useHeadless = true;
@@ -1587,6 +1588,7 @@ export class BrowserManager {
    *   If step 2 fails → return error, headless browser untouched
    */
   async handoff(message: string): Promise<string> {
+    assertHeadedBrowserProvider();
     if (this.connectionMode === 'headed' || this.isHeaded) {
       return `HANDOFF: Already in headed mode at ${this.getCurrentUrl()}`;
     }