fix tmux auto-attach spawning nested clients and blocking detach

Two problems made the auto-attach loop:

$TMUX was the only nesting guard, and it is trivially lost - sudo -i,
env -i, ssh, and dropbear all hand back an interactive tty shell with it
unset, so a shell below tmux would start another tmux. The guard also
never checked for a login shell, so anything spawning `$SHELL -i`
(fzf-tmux, file-manager shell escapes, editor :sh) dragged in a whole
tmux client and a fresh grouped session. Now guarded on a login shell
with stdin and stdout both a terminal, a $TERM that can host tmux, and
an exported $TMUX_AUTOSTART that survives into panes and into nested
shells that strip $TMUX.

detach-on-destroy was off while the per-client sessions set
destroy-unattached on. Those fight: detaching destroys the session, and
off means the client is then switched to the most recently active
remaining session instead of being allowed to leave - so detach put you
back into "main". Restored to on, which is also the default.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mksh/mkshrc

@@ -220,8 +220,20 @@ PS4='[$EPOCHREALTIME] '; PS1='#'; (( USER_ID )) && PS1='→'; PS1=$'\001\r''${|
 # grants each connection its own session grouped with "main" (shared panes,
 # independent window navigation). No -exec, so if tmux is missing or
 # exits/crashes for any reason, this shell just continues normally.
+#
+# Guards, in order: interactive; stdin and stdout are both a terminal; not
+# already inside tmux; not below a tmux we started ($TMUX_AUTOSTART survives
+# into panes and into nested shells that strip $TMUX, e.g. sudo -i, env -i,
+# ssh, dropbear); login shell only, so tools that spawn `$SHELL -i` (fzf-tmux,
+# file-manager shell escapes, editor :sh) don't drag in a whole tmux client;
+# terminal can actually host tmux; tmux is installed.
 tmuxbin=$(\\builtin whence -p tmux 2>/dev/null)
-if [[ $- = *i* ]] && [[ -t 0 ]] && [[ -z $TMUX ]] && [[ -n $tmuxbin ]]; then
+if [[ $- = *i* ]] && [[ -t 0 ]] && [[ -t 1 ]] \
+   && [[ -z $TMUX ]] && [[ -z $TMUX_AUTOSTART ]] \
+   && [[ $0 = -* ]] \
+   && [[ $TERM != dumb && $TERM != linux ]] \
+   && [[ -n $tmuxbin ]]; then
+  \\builtin export TMUX_AUTOSTART=1
   "$tmuxbin"
 fi
 unset tmuxbin

tmux/tmux.conf

@@ -2,7 +2,10 @@
 # set-option -g default-shell /bin/mksh
 set-option -g focus-events on
 set -s escape-time 0
-set -g detach-on-destroy off
+# Must stay `on` (the default): the per-client sessions below set
+# destroy-unattached, so with `off` a detach would destroy the session and then
+# punt the client into "main" instead of letting it leave.
+set -g detach-on-destroy on
 set -g destroy-unattached off
 
 # bare `tmux` (no subcommand, e.g. from mkshrc's auto-attach) gets its own