feat(#1134): add unread matrix messages as a loose end

When the local matrix daemon has rooms with unread notifications,
get_loose_ends now prepends an UnreadMatrix entry to the output so
agents see pending matrix activity alongside questions/reminders
without message content being exposed.

Changes:
- hive-sh4re: add LooseEnd::UnreadMatrix { rooms: u32 } variant
- hive-matrix-mcp: add DaemonRequest::UnreadCount and handler that
  counts joined rooms with notification_count > 0 (server-side push
  notification counts, cached by matrix-sdk)
- hive-ag3nt/mcp: inject UnreadMatrix entry on self-queries by
  connecting to /run/hive-matrix/socket (HIVE_MATRIX_SOCKET override);
  best-effort — agents without matrix configured are unaffected

hive-matrix-mcp/src/socket.rs

@@ -83,5 +83,6 @@ async fn dispatch(req: DaemonRequest, client: &Client) -> DaemonResponse {
         DaemonRequest::JoinRoom { room } => handlers::join_room(client, &room).await,
         DaemonRequest::ListRoomMembers { room } => handlers::list_room_members(client, &room).await,
         DaemonRequest::ReadRoom { room, limit } => handlers::read_room(client, &room, limit).await,
+        DaemonRequest::UnreadCount => handlers::unread_count(client),
     }
 }