feat: netssh multi — several hosts side by side in tmux or zellij

netssh multi opens one pane per host in a tmux window or zellij tab.
Hosts come from names, tag:TAG (NetBox and static hosts), cluster:NAME
(NetBox virtualization clusters; devices by default, --kind vm|all),
and groups from the config (@name, nested, with cycle detection).
Every pane runs netssh HOST, so lookup, rules, and jump hosts apply.

- tmux: a new window inside tmux, else a new session that is attached;
  tiled layout, pane borders with the host and a [SYNC] marker.
- zellij: a generated grid layout; a new tab inside zellij, else a new
  session.
- --sync broadcasts typing to all panes; netssh sync toggles it in the
  current tmux window or zellij tab.
- TUI: Space marks hosts, Ctrl+A marks all, Enter opens them together.
- Configurator: multiplexer, panes per window, groups.

NetBox SearchAll gains a cluster filter (name to cluster IDs per
instance), and cache entries keep the host's cluster for completion
and an offline fallback. Panes skip the 24-hour background refresh
(checked once by multi) and get HOME/XDG paths but no NETSSH_* values,
which would expose tokens in the process list.

testenv/tests/15-multi.sh checks real tmux and zellij sessions,
including a broadcast command arriving on all three cluster nodes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/tui/model.go

@@ -27,8 +27,9 @@ type SelectedHost struct {
 	IP       string
 	User     string   // empty = use config default
 	Port     string   // empty = use default port
-	Action   string   // ActionSSH, ActionSFTP, or ActionTunnel
+	Action   string   // ActionSSH, ActionSFTP, ActionTunnel, or ActionMulti
 	Forwards []string // forward names or specs for ActionTunnel
+	Targets  []string // marked hosts for ActionMulti
 }
 
 // Actions the user can start on a host.
@@ -36,6 +37,7 @@ const (
 	ActionSSH    = ""
 	ActionSFTP   = "sftp"
 	ActionTunnel = "tunnel"
+	ActionMulti  = "multi"
 )
 
 // --- bubbletea messages ---
@@ -74,7 +76,8 @@ func (h hostItem) FilterValue() string { return h.name }
 
 // --- compact list delegate ---
 
-type compactDelegate struct{}
+// compactDelegate shares the model's marks so marked hosts show a dot.
+type compactDelegate struct{ marked map[string]bool }
 
 func (d compactDelegate) Height() int                             { return 1 }
 func (d compactDelegate) Spacing() int                            { return 0 }
@@ -84,7 +87,11 @@ func (d compactDelegate) Render(w io.Writer, m list.Model, index int, item list.
 	if !ok {
 		return
 	}
-	line := fmt.Sprintf("  %s  %s", h.name, lipgloss.NewStyle().Foreground(lipgloss.Color("240")).Render(h.ip))
+	mark := "  "
+	if d.marked[h.key()] {
+		mark = "● "
+	}
+	line := fmt.Sprintf("  %s%s  %s", mark, h.name, lipgloss.NewStyle().Foreground(lipgloss.Color("240")).Render(h.ip))
 	if index == m.Index() {
 		line = lipgloss.NewStyle().Bold(true).Foreground(lipgloss.Color("86")).Render("> " + strings.TrimPrefix(line, "  "))
 	}
@@ -169,10 +176,12 @@ type Model struct {
 	selected                       *SelectedHost
 	width                          int
 	height                         int
-	recentMode                     bool // true when showing recent hosts (empty search, initial state)
-	filterOpen                     bool // Ctrl+F toggles
-	editMode                       bool // Ctrl+E on the selected item
-	forwardMode                    bool // Ctrl+T on the selected item
+	recentMode                     bool            // true when showing recent hosts (empty search, initial state)
+	filterOpen                     bool            // Ctrl+F toggles
+	editMode                       bool            // Ctrl+E on the selected item
+	forwardMode                    bool            // Ctrl+T on the selected item
+	marked                         map[string]bool // hostItem.key() of hosts marked with space
+	markOrder                      []string        // targets in the order they were marked
 }
 
 func New(client *netbox.Client, c *cache.Cache, defaultUser string, defaultPorts ...int) *Model {
@@ -191,7 +200,8 @@ func New(client *netbox.Client, c *cache.Cache, defaultUser string, defaultPorts
 	fwd.Prompt = ""
 	fwd.Placeholder = "web, 8080:localhost:80"
 
-	l := list.New(nil, compactDelegate{}, 0, 0)
+	marked := make(map[string]bool)
+	l := list.New(nil, compactDelegate{marked: marked}, 0, 0)
 	l.SetShowHelp(false)
 	l.SetShowTitle(false)
 	l.SetShowStatusBar(false)
@@ -212,6 +222,7 @@ func New(client *netbox.Client, c *cache.Cache, defaultUser string, defaultPorts
 		filterInput:  fi,
 		editInput:    ei,
 		forwardInput: fwd,
+		marked:       marked,
 		list:         l,
 		client:       client,
 		cache:        c,
@@ -386,11 +397,26 @@ func (m *Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			return m, nil
 
 		case "enter":
+			if len(m.markOrder) > 0 {
+				m.selected = &SelectedHost{Action: ActionMulti, Targets: append([]string(nil), m.markOrder...)}
+				return m, tea.Quit
+			}
 			if item, ok := m.list.SelectedItem().(hostItem); ok {
 				m.selected = &SelectedHost{Name: item.name, IP: item.ip, Kind: item.kind}
 				return m, tea.Quit
 			}
 
+		case "space":
+			if item, ok := m.list.SelectedItem().(hostItem); ok {
+				m.toggleMark(item)
+				m.list.CursorDown()
+				return m, nil
+			}
+
+		case "ctrl+a":
+			m.toggleAllVisible()
+			return m, nil
+
 		case "ctrl+e":
 			if item, ok := m.list.SelectedItem().(hostItem); ok {
 				m.openEdit(item)
@@ -584,7 +610,10 @@ func (m *Model) viewContent() string {
 		sb.WriteString(m.editInput.View() + "\n")
 		sb.WriteString(lipgloss.NewStyle().Foreground(lipgloss.Color("240")).Render("  enter connect  esc cancel") + "\n")
 	} else {
-		hint := "ctrl+c quit  enter connect  ctrl+o sftp  ctrl+t forward  ctrl+e edit  ctrl+f filter  ctrl+n notes  ctrl+b NetBox"
+		hint := "ctrl+c quit  enter connect  space mark  ctrl+o sftp  ctrl+t forward  ctrl+e edit  ctrl+f filter  ctrl+n notes  ctrl+b NetBox"
+		if n := len(m.markOrder); n > 0 {
+			hint = fmt.Sprintf("%d marked — enter open side by side  space unmark  ctrl+a all  esc quit", n)
+		}
 		sb.WriteString("\n" + lipgloss.NewStyle().Foreground(lipgloss.Color("240")).Render(hint) + "\n")
 	}
 
@@ -724,6 +753,42 @@ func (m *Model) buildSelected(item hostItem) *SelectedHost {
 
 func (m *Model) SetNotes(service *notes.Service) { m.noteService = service }
 
+func (h hostItem) key() string { return h.name + "\x00" + h.kind }
+
+// toggleMark marks or unmarks a host for opening several side by side.
+func (m *Model) toggleMark(item hostItem) {
+	key := item.key()
+	if m.marked[key] {
+		delete(m.marked, key)
+		for i, target := range m.markOrder {
+			if target == item.name {
+				m.markOrder = append(m.markOrder[:i], m.markOrder[i+1:]...)
+				break
+			}
+		}
+		return
+	}
+	m.marked[key] = true
+	m.markOrder = append(m.markOrder, item.name)
+}
+
+// toggleAllVisible marks every listed host, or unmarks them if all are marked.
+func (m *Model) toggleAllVisible() {
+	var items []hostItem
+	allMarked := true
+	for _, it := range m.list.Items() {
+		if item, ok := it.(hostItem); ok {
+			items = append(items, item)
+			allMarked = allMarked && m.marked[item.key()]
+		}
+	}
+	for _, item := range items {
+		if m.marked[item.key()] == allMarked {
+			m.toggleMark(item)
+		}
+	}
+}
+
 // SetForwards provides the forward names configured for a host (ctrl+t).
 func (m *Model) SetForwards(forwardsFor func(name, kind string, tags []string) []string) {
 	m.forwardsFor = forwardsFor