open_threads: new get_open_threads MCP tool on agent + manager surfaces

hive-c0re/src/manager_server.rs

@@ -329,6 +329,12 @@ async fn dispatch(req: &ManagerRequest, coord: &Arc<Coordinator>) -> ManagerResp
                 },
             }
         }
+        ManagerRequest::GetOpenThreads => match crate::open_threads::hive_wide(coord) {
+            Ok(threads) => ManagerResponse::OpenThreads { threads },
+            Err(e) => ManagerResponse::Err {
+                message: format!("{e:#}"),
+            },
+        },
     }
 }