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/setup/wizard.go

@@ -91,6 +91,7 @@ func runMenu(cfg *config.Config) error {
 				huh.NewOption("SSH options — jump host, host keys, target, -o, forwards", "ssh"),
 				huh.NewOption(fmt.Sprintf("SSH rules — per tag, instance, kind, name (%d)", len(cfg.SSH.Rules)), "rules"),
 				huh.NewOption(fmt.Sprintf("Static hosts — hosts not in NetBox (%d)", len(cfg.Hosts)), "hosts"),
+				huh.NewOption(fmt.Sprintf("Multi-session — groups, tmux/zellij (%d groups)", len(cfg.Groups)), "multi"),
 				huh.NewOption("Resolver & cache", "resolver"),
 				huh.NewOption("Shortcuts & command alias", "shortcuts"),
 				huh.NewOption("Completion", "completion"),
@@ -115,6 +116,8 @@ func runMenu(cfg *config.Config) error {
 			if hosts, err = configureHosts(cfg.Hosts); err == nil {
 				cfg.Hosts = hosts
 			}
+		case "multi":
+			err = editMulti(cfg)
 		case "resolver":
 			err = editResolverCache(cfg)
 		case "shortcuts":
@@ -369,6 +372,9 @@ func save(cfg config.Config) error {
 	if err := cfg.ValidateHosts(); err != nil {
 		return err
 	}
+	if err := cfg.ValidateMulti(); err != nil {
+		return err
+	}
 	path := config.Path()
 	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
 		return fmt.Errorf("creating config dir: %w", err)
@@ -461,6 +467,31 @@ func save(cfg config.Config) error {
 		}
 	}
 
+	if len(cfg.Groups) > 0 {
+		b.WriteString("\ngroups:\n")
+		names := make([]string, 0, len(cfg.Groups))
+		for name := range cfg.Groups {
+			names = append(names, name)
+		}
+		sort.Strings(names)
+		for _, name := range names {
+			quoted := make([]string, 0, len(cfg.Groups[name]))
+			for _, selector := range cfg.Groups[name] {
+				quoted = append(quoted, strconv.Quote(selector))
+			}
+			fmt.Fprintf(&b, "  %s: [%s]\n", name, strings.Join(quoted, ", "))
+		}
+	}
+	if cfg.Multi.Multiplexer != "" || cfg.Multi.PanesPerWindow > 0 {
+		b.WriteString("\nmulti:\n")
+		if cfg.Multi.Multiplexer != "" {
+			fmt.Fprintf(&b, "  multiplexer: %s\n", cfg.Multi.Multiplexer)
+		}
+		if cfg.Multi.PanesPerWindow > 0 {
+			fmt.Fprintf(&b, "  panes_per_window: %d\n", cfg.Multi.PanesPerWindow)
+		}
+	}
+
 	if cfg.Completion.MaxResults > 0 {
 		fmt.Fprintf(&b, "\ncompletion:\n  max_results: %d\n", cfg.Completion.MaxResults)
 	}