update local startup flow and add root env example

Make the project start more reliably in the current Windows bash setup, add a safe root .env.example for onboarding, and lower the backend Python floor to 3.11 to match the validated local environment.

setup.sh

@@ -83,7 +83,17 @@ require_command npm "Install Node.js and npm first."
 
 if [[ "$USE_WINDOWS_TOOLS" == true ]]; then
   if command -v py.exe >/dev/null 2>&1; then
-    PYTHON_BOOTSTRAP=("py.exe" "-3")
+    if py.exe -3.13 --version >/dev/null 2>&1; then
+      PYTHON_BOOTSTRAP=("py.exe" "-3.13")
+    elif py.exe -3.12 --version >/dev/null 2>&1; then
+      PYTHON_BOOTSTRAP=("py.exe" "-3.12")
+    elif py.exe -3 --version >/dev/null 2>&1; then
+      PYTHON_BOOTSTRAP=("py.exe" "-3")
+    else
+      echo "[ERROR] Python 3 was not found via py.exe."
+      echo "[ERROR] Install Python 3.12+ for Windows first."
+      exit 1
+    fi
   elif command -v python.exe >/dev/null 2>&1; then
     PYTHON_BOOTSTRAP=("python.exe")
   elif command -v python >/dev/null 2>&1; then