feat: align browser callback runtime and export flows

Consolidate the browser task runtime around the callback path, add safer artifact opening for Zhihu exports, and cover the new service/browser flows with focused tests and supporting docs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/common/mod.rs

@@ -17,6 +17,7 @@ impl MockTransport {
         }
     }
 
+    #[allow(dead_code)]
     pub fn sent_messages(&self) -> Vec<AgentMessage> {
         self.sent.lock().unwrap().clone()
     }