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/multiplex/tmux.go

@@ -1,6 +1,8 @@
 package multiplex
 
 import (
+	"fmt"
+	"path/filepath"
 	"strconv"
 	"strings"
 )
@@ -70,8 +72,9 @@ func openTmux(panes []Pane, opts Options, r Runner) error {
 			_, _ = r.Output("tmux", "set-option", "-p", "-t", id, "@netssh_host", group[j].Title)
 		}
 		windowOptions := [][]string{
+			{"@netssh", "1"}, // the key bindings act only in these windows
 			{"pane-border-status", "top"},
-			{"pane-border-format", " #{@netssh_host}#{?pane_synchronized, [SYNC],} "},
+			{"pane-border-format", " #{@netssh_host}#{?pane_synchronized, [SYNC],#{?@netssh_solo, [SOLO],}} "},
 		}
 		if opts.Sync {
 			windowOptions = append(windowOptions, []string{"synchronize-panes", "on"})
@@ -82,14 +85,103 @@ func openTmux(panes []Pane, opts Options, r Runner) error {
 			}
 		}
 	}
+	keys, err := bindKeys(r, opts)
+	if err != nil {
+		return err
+	}
+	if keys != "" {
+		opts.Hint(keys)
+	}
 	if inside {
 		_, err := r.Output("tmux", "select-window", "-t", first)
 		return err
 	}
-	opts.Hint("toggle broadcast with: netssh sync (or Ctrl+b :setw synchronize-panes)")
 	return r.Exec("tmux", "attach-session", "-t", session)
 }
 
+// soloToggle excludes the current pane from broadcast (a pane-level
+// synchronize-panes off) or includes it again by removing that override.
+func soloToggle(solo bool) [][]string {
+	if solo {
+		return [][]string{
+			{"tmux", "set-option", "-pu", "synchronize-panes"},
+			{"tmux", "set-option", "-pu", "@netssh_solo"},
+		}
+	}
+	return [][]string{
+		{"tmux", "set-option", "-p", "synchronize-panes", "off"},
+		{"tmux", "set-option", "-p", "@netssh_solo", "1"},
+	}
+}
+
+// keyBindings toggle broadcast in netssh multi windows; elsewhere they only
+// show a message, so they do not change what the keys mean to other windows.
+const syncBinding = `bind-key -T prefix %s if-shell -F '#{@netssh}' {
+  set-window-option synchronize-panes
+  display-message 'netssh: broadcast #{?synchronize-panes,on,off}'
+} {
+  display-message 'netssh: not a netssh multi window'
+}
+`
+
+const soloBinding = `bind-key -T prefix %s if-shell -F '#{@netssh}' {
+  if-shell -F '#{@netssh_solo}' {
+    set-option -pu synchronize-panes
+    set-option -pu @netssh_solo
+    display-message 'netssh: pane included in broadcast'
+  } {
+    set-option -p synchronize-panes off
+    set-option -p @netssh_solo 1
+    display-message 'netssh: pane excluded from broadcast'
+  }
+} {
+  display-message 'netssh: not a netssh multi window'
+}
+`
+
+// bindKeys installs the broadcast keys in the tmux server unless the user's
+// own configuration already uses them, and returns a hint naming the keys.
+func bindKeys(r Runner, opts Options) (string, error) {
+	prefix, err := r.Output("tmux", "show-options", "-gv", "prefix")
+	if err != nil || prefix == "" {
+		prefix = "C-b"
+	}
+	var conf strings.Builder
+	var hints, taken []string
+	for _, key := range []struct{ key, binding, what string }{
+		{opts.SyncKey, syncBinding, "broadcast to all panes"},
+		{opts.SoloKey, soloBinding, "exclude/include this pane"},
+	} {
+		if key.key == "" {
+			continue
+		}
+		// An existing binding is replaced only if netssh made it.
+		if existing, _ := r.Output("tmux", "list-keys", "-T", "prefix", key.key); existing != "" && !strings.Contains(existing, "@netssh") {
+			taken = append(taken, prefix+" "+key.key)
+			continue
+		}
+		fmt.Fprintf(&conf, key.binding, key.key)
+		hints = append(hints, prefix+" "+key.key+" "+key.what)
+	}
+	if conf.Len() > 0 {
+		path := filepath.Join(opts.LayoutDir, "netssh-multi.tmux.conf")
+		if err := r.WriteFile(path, []byte(conf.String())); err != nil {
+			return "", err
+		}
+		if _, err := r.Output("tmux", "source-file", path); err != nil {
+			return "", err
+		}
+	}
+	hint := "keys: " + strings.Join(hints, ", ")
+	if len(hints) == 0 {
+		hint = "toggle broadcast with: netssh sync, netssh sync --pane"
+	}
+	if len(taken) > 0 {
+		hint += "; " + strings.Join(taken, " and ") + " kept your own binding (use netssh sync)"
+	}
+	return hint, nil
+}
+
 // freeSession returns name, or name-2, name-3, … if a session already uses it.
 func freeSession(r Runner, name string) string {
 	candidate := name