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>

src/runtime/engine.rs

@@ -179,17 +179,6 @@ impl RuntimeEngine {
         skills
     }
 
-    pub fn loaded_skill_names(&self, config: &ZeroClawConfig, skills_dir: &Path) -> Vec<String> {
-        let mut names = self
-            .loaded_skills(config, skills_dir)
-            .into_iter()
-            .map(|skill| skill.name)
-            .collect::<Vec<_>>();
-        names.sort();
-        names.dedup();
-        names
-    }
-
     pub fn should_attach_openxml_office_tool(&self, instruction: &str) -> bool {
         task_needs_office_export(instruction)
     }