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_test.go

@@ -122,3 +122,38 @@ func TestForwardInputEscCancels(t *testing.T) {
 		t.Fatal("esc did not cancel")
 	}
 }
+
+func TestSpaceMarksHostsAndEnterOpensThem(t *testing.T) {
+	m := New(nil, nil, "")
+	m.updateListItems([]netbox.HostEntry{{Name: "a", Kind: "device"}, {Name: "b", Kind: "device"}, {Name: "c", Kind: "vm"}})
+	m.Update(tea.KeyPressMsg{Code: tea.KeySpace, Text: " "})
+	if len(m.markOrder) != 1 || m.markOrder[0] != "a" || m.list.Index() != 1 {
+		t.Fatalf("space: marks %v, cursor %d", m.markOrder, m.list.Index())
+	}
+	if m.input.Value() != "" {
+		t.Fatal("space went into the search field")
+	}
+	m.list.Select(2)
+	m.Update(tea.KeyPressMsg{Code: tea.KeySpace, Text: " "})
+	if !strings.Contains(m.viewContent(), "2 marked") {
+		t.Fatalf("no marked count:\n%s", m.viewContent())
+	}
+	m.Update(tea.KeyPressMsg{Code: tea.KeyEnter})
+	if m.selected == nil || m.selected.Action != ActionMulti || !reflect.DeepEqual(m.selected.Targets, []string{"a", "c"}) {
+		t.Fatalf("got %+v", m.selected)
+	}
+}
+
+func TestCtrlATogglesAllVisible(t *testing.T) {
+	m := New(nil, nil, "")
+	m.updateListItems([]netbox.HostEntry{{Name: "a"}, {Name: "b"}})
+	m.toggleMark(hostItem{name: "a"})
+	m.Update(tea.KeyPressMsg{Code: 'a', Mod: tea.ModCtrl})
+	if !reflect.DeepEqual(m.markOrder, []string{"a", "b"}) {
+		t.Fatalf("mark all: %v", m.markOrder)
+	}
+	m.Update(tea.KeyPressMsg{Code: 'a', Mod: tea.ModCtrl})
+	if len(m.markOrder) != 0 {
+		t.Fatalf("unmark all: %v", m.markOrder)
+	}
+}