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/multi.go

@@ -0,0 +1,102 @@
+package setup
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+
+	"charm.land/huh/v2"
+
+	"git.zb-server.de/Sebi/ssh-netbox-wrapper/internal/config"
+)
+
+func editMulti(cfg *config.Config) error {
+	multiplexer := cfg.Multi.Multiplexer
+	if multiplexer == "" {
+		multiplexer = "auto"
+	}
+	perWindow := ""
+	if cfg.Multi.PanesPerWindow > 0 {
+		perWindow = strconv.Itoa(cfg.Multi.PanesPerWindow)
+	}
+	groups := formatGroups(cfg.Groups)
+	err := huh.NewForm(huh.NewGroup(
+		huh.NewSelect[string]().
+			Title("Multiplexer").
+			Description("netssh multi opens hosts side by side. auto uses the one you are in, else tmux, else zellij.").
+			Options(huh.NewOption("auto", "auto"), huh.NewOption("tmux", "tmux"), huh.NewOption("zellij", "zellij")).
+			Value(&multiplexer),
+		huh.NewInput().
+			Title("Panes per window").
+			Description(fmt.Sprintf("More hosts open in further windows or tabs. Leave empty for %d.", config.DefaultPanesPerWindow)).
+			Value(&perWindow).
+			Validate(func(s string) error {
+				if s = strings.TrimSpace(s); s == "" {
+					return nil
+				}
+				if n, err := strconv.Atoi(s); err != nil || n < 1 || n > 64 {
+					return errors.New("must be a number between 1 and 64")
+				}
+				return nil
+			}),
+		huh.NewText().
+			Title("Groups").
+			Description("One per line: NAME: host, tag:TAG, cluster:NAME, @other — open with netssh multi @NAME.").
+			Lines(4).
+			Value(&groups).
+			Validate(func(s string) error {
+				_, err := parseGroups(s)
+				return err
+			}),
+	).Title("Multi-session")).Run()
+	if err != nil {
+		return err
+	}
+	cfg.Multi.Multiplexer = multiplexer
+	if multiplexer == "auto" {
+		cfg.Multi.Multiplexer = ""
+	}
+	cfg.Multi.PanesPerWindow, _ = strconv.Atoi(strings.TrimSpace(perWindow))
+	cfg.Groups, _ = parseGroups(groups)
+	return nil
+}
+
+// parseGroups reads "NAME: sel1, sel2" lines into groups:.
+func parseGroups(s string) (map[string][]string, error) {
+	var out map[string][]string
+	for _, line := range parseLines(s) {
+		name, members, ok := strings.Cut(line, ":")
+		name = strings.ToLower(strings.TrimSpace(name))
+		// "tag:x" alone has no group name before the first colon worth keeping.
+		if !ok || name == "" || name == "tag" || name == "cluster" {
+			return nil, fmt.Errorf("%q: expected NAME: host, tag:TAG, …", line)
+		}
+		var selectors []string
+		for _, field := range strings.FieldsFunc(members, func(r rune) bool { return r == ',' || r == ' ' || r == '\t' }) {
+			selectors = append(selectors, field)
+		}
+		if out == nil {
+			out = make(map[string][]string)
+		}
+		out[name] = append(out[name], selectors...)
+	}
+	if err := (&config.Config{Groups: out}).ValidateMulti(); err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func formatGroups(groups map[string][]string) string {
+	names := make([]string, 0, len(groups))
+	for name := range groups {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	lines := make([]string, 0, len(names))
+	for _, name := range names {
+		lines = append(lines, name+": "+strings.Join(groups[name], ", "))
+	}
+	return strings.Join(lines, "\n")
+}