feat(tools): add welcome message (MOTD) management tool

- add `tools/welcome-message.sh` with view, set, and reset actions
- register `lxs tool welcome|motd` command in `lxs.sh`
- add option 7 to interactive tools menu in `tools/index.sh`
- document `lxs tool welcome` in README

lxs.sh

@@ -290,6 +290,7 @@ cmd_tool() {
         root-password) download_and_run "tools/root-password.sh" "$@" ;;
         update)    download_and_run "tools/update-server.sh" "$@" ;;
         root-ssh-login) download_and_run "tools/root-ssh-login.sh" "$@" ;;
+        welcome|motd) download_and_run "tools/welcome-message.sh" "$@" ;;
         "")        echo -e "${RED}[✗] Missing tool name. Try: lxs help${NC}"; return 1 ;;
         *)         echo -e "${RED}[✗] Unknown tool: $tool. Try: lxs help${NC}"; return 1 ;;
     esac