fix: auto-source when helper functions unavailable

The wt function now re-sources ~/.wt/wt.sh if helper functions
are missing. Fixes usage in environments where only the main
function is available but dependencies are not.

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

Author: Test User

wt.sh

@@ -733,7 +733,7 @@ _wt_update() {
   fi
 }
 
-wt() {
+_wt_main() {
   _wt_setup_colors
   case "$1" in
     new) shift; _wt_new "$@" ;;
@@ -757,3 +757,10 @@ wt() {
       ;;
   esac
 }
+
+wt() {
+  if ! type _wt_new &>/dev/null; then
+    source "$HOME/.wt/wt.sh"
+  fi
+  _wt_main "$@"
+}