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/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)
 		}