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

pkg/tui/host_list_tab.go

@@ -56,7 +56,15 @@ func (t *HostListTab) Update(msg tea.Msg) (Tab, tea.Cmd) {
 
 		case "enter", " ":
 			if len(t.hosts) > 0 {
-				return t, tea.Quit
+				host := t.hosts[t.selectedIndex]
+				return t, func() tea.Msg {
+					return openSessionMsg{host: host}
+				}
+			}
+
+		case "q", "ctrl+c":
+			return t, func() tea.Msg {
+				return quitMsg{}
 			}
 		}
 	}
@@ -89,6 +97,9 @@ func (t *HostListTab) View() string {
 	return b.String()
 }
 
+// Close is a no-op for host list tab
+func (t *HostListTab) Close() {}
+
 // SetHosts sets the host list
 func (t *HostListTab) SetHosts(hosts []*models.Host) {
 	t.hosts = hosts