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/setup/wizard.go

@@ -482,7 +482,7 @@ func save(cfg config.Config) error {
 			fmt.Fprintf(&b, "  %s: [%s]\n", name, strings.Join(quoted, ", "))
 		}
 	}
-	if cfg.Multi.Multiplexer != "" || cfg.Multi.PanesPerWindow > 0 {
+	if cfg.Multi != (config.MultiConfig{}) {
 		b.WriteString("\nmulti:\n")
 		if cfg.Multi.Multiplexer != "" {
 			fmt.Fprintf(&b, "  multiplexer: %s\n", cfg.Multi.Multiplexer)
@@ -490,6 +490,11 @@ func save(cfg config.Config) error {
 		if cfg.Multi.PanesPerWindow > 0 {
 			fmt.Fprintf(&b, "  panes_per_window: %d\n", cfg.Multi.PanesPerWindow)
 		}
+		for _, key := range [][2]string{{"sync_key", cfg.Multi.SyncKey}, {"solo_key", cfg.Multi.SoloKey}} {
+			if key[1] != "" {
+				fmt.Fprintf(&b, "  %s: %q\n", key[0], key[1])
+			}
+		}
 	}
 
 	if cfg.Completion.MaxResults > 0 {