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/netbox/models.go

@@ -7,6 +7,8 @@ import "strings"
 type SearchOptions struct {
 	Tag  string // filter by tag slug; empty = no filter
 	Kind string // "device" | "vm" | "" (both)
+	// Cluster filters SearchAll by virtualization cluster name (case-insensitive).
+	Cluster string
 }
 
 // HostEntry is a unified model for both devices and virtual machines from NetBox.
@@ -18,6 +20,13 @@ type HostEntry struct {
 	PrimaryIP4 string // e.g. "10.0.1.5" (prefix length stripped)
 	PrimaryIP6 string
 	Tags       []string
+	Cluster    string // virtualization cluster name, if assigned
+}
+
+// netboxCluster is the nested cluster of a device or virtual machine.
+type netboxCluster struct {
+	ID   int    `json:"id"`
+	Name string `json:"name"`
 }
 
 // netboxIP represents an IP address as returned by the NetBox API.
@@ -32,8 +41,9 @@ type netboxDevice struct {
 	Tags []struct {
 		Name string `json:"name"`
 	} `json:"tags"`
-	PrimaryIP4 *netboxIP `json:"primary_ip4"`
-	PrimaryIP6 *netboxIP `json:"primary_ip6"`
+	PrimaryIP4 *netboxIP      `json:"primary_ip4"`
+	PrimaryIP6 *netboxIP      `json:"primary_ip6"`
+	Cluster    *netboxCluster `json:"cluster"`
 }
 
 // netboxVM matches the relevant fields of the NetBox /virtualization/virtual-machines/ response.
@@ -43,8 +53,9 @@ type netboxVM struct {
 	Tags []struct {
 		Name string `json:"name"`
 	} `json:"tags"`
-	PrimaryIP4 *netboxIP `json:"primary_ip4"`
-	PrimaryIP6 *netboxIP `json:"primary_ip6"`
+	PrimaryIP4 *netboxIP      `json:"primary_ip4"`
+	PrimaryIP6 *netboxIP      `json:"primary_ip6"`
+	Cluster    *netboxCluster `json:"cluster"`
 }
 
 type netboxListResponse[T any] struct {