manager: add optional agent param to GetLooseEnds

GetLooseEnds now takes agent: Option<String>:
- None   = manager's own loose ends (default; the bug fix)
- Some("*")    = hive-wide view (every approval/question/reminder)
- Some("name") = that agent's loose ends

The get_loose_ends MCP tool exposes this as an optional agent arg, so
the manager can still scan the whole swarm on demand. The web UI and
post-turn counts pass None (manager's own).

hive-ag3nt/src/bin/hive-m1nd.rs

@@ -334,7 +334,7 @@ fn now_unix() -> i64 {
 async fn fetch_manager_post_turn_counts(socket: &Path) -> (Option<u64>, Option<u64>) {
     let threads = match client::request::<_, ManagerResponse>(
         socket,
-        &ManagerRequest::GetLooseEnds,
+        &ManagerRequest::GetLooseEnds { agent: None },
     )
     .await
     {