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/client.go

@@ -105,6 +105,22 @@ func (c *Client) SearchAll(ctx context.Context, query string, opts SearchOptions
 	if opts.Kind != "" && opts.Kind != "device" && opts.Kind != "vm" {
 		return nil, fmt.Errorf("invalid host kind %q", opts.Kind)
 	}
+	filters := ""
+	if opts.Tag != "" {
+		filters += "&tag=" + url.QueryEscape(opts.Tag)
+	}
+	if opts.Cluster != "" {
+		ids, err := c.clusterIDs(ctx, opts.Cluster)
+		if err != nil {
+			return nil, fmt.Errorf("clusters: %w", err)
+		}
+		if len(ids) == 0 {
+			return nil, nil // no such cluster in this NetBox
+		}
+		for _, id := range ids {
+			filters += fmt.Sprintf("&cluster_id=%d", id)
+		}
+	}
 	var (
 		mu      sync.Mutex
 		results []HostEntry
@@ -116,7 +132,7 @@ func (c *Client) SearchAll(ctx context.Context, query string, opts SearchOptions
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			devices, err := c.fetchAllDevices(ctx, query, opts.Tag)
+			devices, err := c.fetchAllDevices(ctx, query, filters)
 			mu.Lock()
 			defer mu.Unlock()
 			if err != nil {
@@ -131,7 +147,7 @@ func (c *Client) SearchAll(ctx context.Context, query string, opts SearchOptions
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			vms, err := c.fetchAllVMs(ctx, query, opts.Tag)
+			vms, err := c.fetchAllVMs(ctx, query, filters)
 			mu.Lock()
 			defer mu.Unlock()
 			if err != nil {
@@ -231,15 +247,27 @@ func (c *Client) searchVMs(ctx context.Context, query, tag string) ([]HostEntry,
 	return entries, nil
 }
 
-func (c *Client) fetchAllDevices(ctx context.Context, query, tag string) ([]HostEntry, error) {
+// clusterIDs returns the IDs of the clusters named name (case-insensitive).
+func (c *Client) clusterIDs(ctx context.Context, name string) ([]int, error) {
+	var resp netboxListResponse[netboxCluster]
+	apiURL := fmt.Sprintf("%s/api/virtualization/clusters/?name__ie=%s&limit=50", c.baseURL, url.QueryEscape(name))
+	if err := c.get(ctx, apiURL, &resp); err != nil {
+		return nil, err
+	}
+	ids := make([]int, 0, len(resp.Results))
+	for _, cluster := range resp.Results {
+		ids = append(ids, cluster.ID)
+	}
+	return ids, nil
+}
+
+// fetchAllDevices pages through devices; filters are extra "&key=value" parameters.
+func (c *Client) fetchAllDevices(ctx context.Context, query, filters string) ([]HostEntry, error) {
 	const pageSize = 50
 	var all []HostEntry
 	for offset := 0; ; offset += pageSize {
 		apiURL := fmt.Sprintf("%s/api/dcim/devices/?name__ic=%s&limit=%d&offset=%d",
-			c.baseURL, url.QueryEscape(query), pageSize, offset)
-		if tag != "" {
-			apiURL += "&tag=" + url.QueryEscape(tag)
-		}
+			c.baseURL, url.QueryEscape(query), pageSize, offset) + filters
 		var resp netboxListResponse[netboxDevice]
 		if err := c.get(ctx, apiURL, &resp); err != nil {
 			return nil, err
@@ -254,15 +282,12 @@ func (c *Client) fetchAllDevices(ctx context.Context, query, tag string) ([]Host
 	return all, nil
 }
 
-func (c *Client) fetchAllVMs(ctx context.Context, query, tag string) ([]HostEntry, error) {
+func (c *Client) fetchAllVMs(ctx context.Context, query, filters string) ([]HostEntry, error) {
 	const pageSize = 50
 	var all []HostEntry
 	for offset := 0; ; offset += pageSize {
 		apiURL := fmt.Sprintf("%s/api/virtualization/virtual-machines/?name__ic=%s&limit=%d&offset=%d",
-			c.baseURL, url.QueryEscape(query), pageSize, offset)
-		if tag != "" {
-			apiURL += "&tag=" + url.QueryEscape(tag)
-		}
+			c.baseURL, url.QueryEscape(query), pageSize, offset) + filters
 		var resp netboxListResponse[netboxVM]
 		if err := c.get(ctx, apiURL, &resp); err != nil {
 			return nil, err
@@ -343,6 +368,9 @@ func deviceToEntry(d netboxDevice) HostEntry {
 	for _, t := range d.Tags {
 		e.Tags = append(e.Tags, t.Name)
 	}
+	if d.Cluster != nil {
+		e.Cluster = d.Cluster.Name
+	}
 	return e
 }
 
@@ -357,6 +385,9 @@ func vmToEntry(v netboxVM) HostEntry {
 	for _, t := range v.Tags {
 		e.Tags = append(e.Tags, t.Name)
 	}
+	if v.Cluster != nil {
+		e.Cluster = v.Cluster.Name
+	}
 	return e
 }