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>

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