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>

cmd/netssh/multi.go

@@ -83,12 +83,19 @@ netssh sync. Inside tmux or zellij the panes open in the current session.`,
 }
 
 func syncCmd() *cobra.Command {
-	return &cobra.Command{
-		Use:   "sync",
+	var pane bool
+	cmd := &cobra.Command{
+		Use:   "sync [--pane]",
 		Short: "Toggle typing into all panes of the current tmux window or zellij tab",
-		Args:  cobra.NoArgs,
+		Long: `Toggle broadcast in the current tmux window or zellij tab. With --pane,
+exclude the current tmux pane from broadcast, or include it again.
+
+In windows opened by netssh multi, tmux also has keys for this:
+prefix S (all panes) and prefix e (this pane); see multi.sync_key and
+multi.solo_key. In zellij, use Ctrl+t then s.`,
+		Args: cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			state, err := multiplex.ToggleSync(multiplex.System{})
+			state, err := multiplex.ToggleSync(multiplex.System{}, pane)
 			if err != nil {
 				return err
 			}
@@ -96,6 +103,8 @@ func syncCmd() *cobra.Command {
 			return nil
 		},
 	}
+	cmd.Flags().BoolVar(&pane, "pane", false, "Exclude the current tmux pane from broadcast, or include it again")
+	return cmd
 }
 
 // runMulti expands selectors and opens one pane per host.
@@ -147,7 +156,10 @@ func runMulti(cfg *config.Config, selectors []string, f multiFlags) error {
 	if err := os.MkdirAll(layoutDir, 0o700); err != nil {
 		return err
 	}
+	syncKey, soloKey := cfg.Multi.Keys()
 	return multiplex.Open(kind, panes, multiplex.Options{
+		SyncKey:   syncKey,
+		SoloKey:   soloKey,
 		Label:     multiLabel(selectors),
 		Sync:      f.sync,
 		PerWindow: cfg.Multi.PanesPerWindowOrDefault(),