mksh: auto-attach a shared tmux session per host on login

Guarded to only fire for interactive shells with a real tty, when
tmux isn't already active, and only if the tmux binary is found -
so it falls back to a plain shell if any of that isn't true. Not
exec'd, so an unexpected tmux exit also drops back to a normal shell
instead of closing the connection. Replaces manually running bin/tmx.

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

mksh/mkshrc

@@ -215,3 +215,11 @@ PS4='[$EPOCHREALTIME] '; PS1='#'; (( USER_ID )) && PS1='→'; PS1=$'\001\r''${|
 
 	\\builtin return $e
 } '"${hostcol}${bld}$PS1 ${rst}"
+
+# tmux auto-attach: one shared session per host. No -exec, so if tmux is
+# missing or exits/crashes for any reason, this shell just continues normally.
+tmuxbin=$(\\builtin whence -p tmux 2>/dev/null)
+if [[ $- = *i* ]] && [[ -t 0 ]] && [[ -z $TMUX ]] && [[ -n $tmuxbin ]]; then
+  "$tmuxbin" new-session -A -s "${HOSTNAME:-main}"
+fi
+unset tmuxbin