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/browser_script_skill_tool_test.rs

@@ -9,6 +9,7 @@ use std::time::{SystemTime, UNIX_EPOCH};
 
 use common::MockTransport;
 use serde_json::json;
+use sgclaw::browser::{BrowserBackend, PipeBrowserBackend};
 use sgclaw::compat::browser_script_skill_tool::BrowserScriptSkillTool;
 use sgclaw::pipe::{Action, AgentMessage, BrowserMessage, BrowserPipeTool, Timing};
 use sgclaw::security::MacPolicy;
@@ -67,6 +68,7 @@ return {
         vec![1, 2, 3, 4, 5, 6, 7, 8],
     )
     .with_response_timeout(Duration::from_secs(1));
+    let backend: Arc<dyn BrowserBackend> = Arc::new(PipeBrowserBackend::from_inner(browser_tool));
 
     let mut args = HashMap::new();
     args.insert("top_n".to_string(), "How many rows to extract".to_string());
@@ -77,7 +79,7 @@ return {
         command: "scripts/extract_hotlist.js".to_string(),
         args,
     };
-    let tool = BrowserScriptSkillTool::new("zhihu-hotlist", &skill_tool, &skill_dir, browser_tool)
+    let tool = BrowserScriptSkillTool::new("zhihu-hotlist", &skill_tool, &skill_dir, backend)
         .unwrap();
 
     let result = tool