implement broadcast messaging: send to '*' reaches all agents with hint

hive-c0re/src/coordinator.rs

@@ -117,6 +117,9 @@ impl Coordinator {
             let _ = std::fs::remove_file(&socket.path);
         }
     }
+    pub fn list_agents(&self) -> Vec<String> {
+        self.agents.lock().unwrap().keys().cloned().collect()
+    }
 
     /// Mark an agent as in-progress (only one state per agent for now).
     pub fn set_transient(&self, name: &str, kind: TransientKind) {