feat: Task 16 — SSH Session Tab (multi-session)

- pkg/tui/session.go — SessionTab with live SSH terminal in a tab
- pkg/tui/messages.go — quitMsg, openSessionMsg, sessionOutputMsg
- pkg/tui/tabs.go — Tab.Close() interface, Add() returns tea.Cmd
- pkg/tui/host_list_tab.go — Enter opens session tab, q quits
- pkg/tui/tui.go — handle openSessionMsg/quitMsg, SetDataDir
- cmd/hostkeeper/tui.go — pass dataDir to model

cmd/hostkeeper/tui.go

@@ -46,6 +46,7 @@ func runTUI(cmd *cobra.Command, args []string) error {
 	}
 
 	model := tui.New()
+	model.SetDataDir(cfg.GetDataDir())
 	model.LoadHosts(hosts)
 
 	p := tea.NewProgram(model)