feat: add tmux multip window & split panel right support

[ipumpkin]

feat: add tmux window/pane tree hierarchy and horizontal split command

• Add TmuxWindowItem and TmuxPaneItem classes to the tree view provider,
  exposing per-window and per-pane details under TmuxDetailItem.
• Make TmuxDetailItem collapsible (Collapsed by default) to host child
  window nodes; each window node expands to show its panes.
• Implement listWindows() and listPanes() in TmuxBackend using
  list-windows and list-panes format queries; ZellijBackend stubs
  return empty arrays.
• Add optional windowIndex parameter to MultiplexerBackend.attachSession()
  so clicking a window/pane item attaches directly to that target.
• Extend splitPane() with a direction parameter ('vertical' | 'horizontal')
  and expose a new tmux.newPaneHorizontal command with context menu entry
  for right-side splits.
• Update contextMenu.ts and removeTask.ts to dispatch on the new item types.
• Label split commands as "Split Pane (Down)" and "Split Pane (Right)" for
  clarity.