nixos/tmux: use idempotent new-session to avoid duplicate sessions (#468337)

    setw -g pane-base-index ${toString cfg.baseIndex}

    set  -g history-limit   ${toString cfg.historyLimit}

    ${optionalString cfg.newSession "new-session"}

    ${optionalString cfg.newSession ''

      # Use -A to make new-session idempotent: attach if session "0" exists,

      # otherwise create it. This prevents duplicate sessions when multiple

      # configs (e.g., system and user) both enable newSession.

      new-session -A -s 0

    ''}

    ${optionalString cfg.reverseSplit ''

      bind v split-window -h