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_test.go

@@ -617,3 +617,37 @@ func TestValidateHostIn(t *testing.T) {
 		t.Fatal("invalid address accepted")
 	}
 }
+
+func TestParseGroupsAndSaveMulti(t *testing.T) {
+	groups, err := parseGroups("K8s: master1, tag:k8s-worker\npve: cluster:pve-fsn1 @k8s\n")
+	want := map[string][]string{"k8s": {"master1", "tag:k8s-worker"}, "pve": {"cluster:pve-fsn1", "@k8s"}}
+	if err != nil || !reflect.DeepEqual(groups, want) {
+		t.Fatalf("got %v %v", groups, err)
+	}
+	if again, _ := parseGroups(formatGroups(groups)); !reflect.DeepEqual(again, want) {
+		t.Fatalf("roundtrip: %v", again)
+	}
+	for _, bad := range []string{"no colon", "tag:x", "bad name: a", "empty:"} {
+		if _, err := parseGroups(bad); err == nil {
+			t.Errorf("%q accepted", bad)
+		}
+	}
+
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	t.Setenv("HOME", t.TempDir())
+	cfg := config.Config{
+		Hosts:  []config.StaticHost{{Name: "a", Address: "10.0.0.1"}},
+		Groups: groups,
+		Multi:  config.MultiConfig{Multiplexer: "zellij", PanesPerWindow: 4},
+	}
+	if err := save(cfg); err != nil {
+		t.Fatal(err)
+	}
+	loaded, err := config.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(loaded.Groups, want) || loaded.Multi != cfg.Multi {
+		t.Fatalf("got %v %+v", loaded.Groups, loaded.Multi)
+	}
+}