feat(multi): tmux keys to toggle broadcast and exclude single panes

In windows opened by netssh multi, prefix S toggles broadcast for all
panes and prefix e excludes the current pane (a pane-level
synchronize-panes off, shown as [SOLO] in the border) or includes it
again. netssh sync --pane does the same from a shell.

S and e are unbound in a default tmux (E would be "spread evenly").
netssh writes the bindings to a small tmux config and sources it, binds
a key only if the user's tmux.conf does not use it (netssh's own earlier
binding is refreshed), and the bindings act only in windows marked
@netssh. multi.sync_key and multi.solo_key change the keys or turn them
off (none); also in the configurator. zellij syncs whole tabs only, so
excluding a pane is tmux-only.

testenv: the render test presses the keys through the xterm client
(send-keys -K) and checks [SOLO], that broadcast skips the excluded
pane, that S toggles, and that other windows are unaffected. The client
gets GNU grep: BusyBox grep -oF counted only one match per line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/config/multi.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"regexp"
 	"sort"
 	"strings"
 )
@@ -12,8 +13,34 @@ type MultiConfig struct {
 	Multiplexer string `mapstructure:"multiplexer"`
 	// PanesPerWindow splits larger host lists into more windows or tabs; 0 = 9.
 	PanesPerWindow int `mapstructure:"panes_per_window"`
+	// SyncKey and SoloKey are tmux prefix keys for broadcast to all panes and
+	// for excluding the current pane; empty = default (S, e), "none" = off.
+	SyncKey string `mapstructure:"sync_key"`
+	SoloKey string `mapstructure:"solo_key"`
 }
 
+// Default tmux keys: unbound in a stock tmux (E would be "spread evenly").
+const (
+	DefaultSyncKey = "S"
+	DefaultSoloKey = "e"
+)
+
+// Keys returns the tmux keys to bind; "" for a disabled one.
+func (m MultiConfig) Keys() (sync, solo string) {
+	pick := func(key, fallback string) string {
+		switch key {
+		case "":
+			return fallback
+		case "none":
+			return ""
+		}
+		return key
+	}
+	return pick(m.SyncKey, DefaultSyncKey), pick(m.SoloKey, DefaultSoloKey)
+}
+
+var tmuxKey = regexp.MustCompile(`^([CMS]-)*([A-Za-z0-9]|F[0-9]{1,2}|[-_=+!@#$%^&*,./?<>\[\]]|Tab|Space|Enter|BSpace|Up|Down|Left|Right|Home|End|PPage|NPage)$`)
+
 // DefaultPanesPerWindow keeps panes readable on a normal screen (3×3).
 const DefaultPanesPerWindow = 9
 
@@ -27,6 +54,14 @@ func (c *Config) ValidateMulti() error {
 	if c.Multi.PanesPerWindow < 0 || c.Multi.PanesPerWindow > 64 {
 		return fmt.Errorf("multi.panes_per_window must be between 1 and 64")
 	}
+	for name, key := range map[string]string{"sync_key": c.Multi.SyncKey, "solo_key": c.Multi.SoloKey} {
+		if key != "" && key != "none" && !tmuxKey.MatchString(key) {
+			return fmt.Errorf("multi.%s %q is not a tmux key (e.g. S, e, C-s, F5) or none", name, key)
+		}
+	}
+	if sync, solo := c.Multi.Keys(); sync != "" && sync == solo {
+		return fmt.Errorf("multi.sync_key and multi.solo_key must differ")
+	}
 	names := make([]string, 0, len(c.Groups))
 	for name := range c.Groups {
 		names = append(names, name)

internal/config/ssh_test.go

@@ -196,7 +196,15 @@ func TestMultiConfig(t *testing.T) {
 	if err != nil || len(cfg.Groups["k8s"]) != 2 || cfg.Multi.Multiplexer != "zellij" || cfg.Multi.PanesPerWindowOrDefault() != 9 {
 		t.Fatalf("%+v %v", cfg, err)
 	}
-	for _, bad := range []string{"multi: {multiplexer: screen}", "multi: {panes_per_window: 100}", "groups: {web: []}", "groups: {web: [\"a b\"]}"} {
+	if sync, solo := cfg.Multi.Keys(); sync != "S" || solo != "e" {
+		t.Fatalf("default keys %q %q", sync, solo)
+	}
+	custom, err := loadYAML(t, "multi: {sync_key: C-s, solo_key: none}\n")
+	if sync, solo := custom.Multi.Keys(); err != nil || sync != "C-s" || solo != "" {
+		t.Fatalf("custom keys %q %q %v", sync, solo, err)
+	}
+	for _, bad := range []string{"multi: {multiplexer: screen}", "multi: {panes_per_window: 100}", "groups: {web: []}", "groups: {web: [\"a b\"]}",
+		"multi: {sync_key: \"x y\"}", "multi: {sync_key: \"{\"}", "multi: {sync_key: e}"} {
 		if _, err := loadYAML(t, bad+"\n"); err == nil {
 			t.Errorf("%s accepted", bad)
 		}