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

README.md

@@ -53,6 +53,7 @@ lxs help                  # Show help
 | `lxs tool root-password` | Change the root password (interactive or generated) |
 | `lxs tool update` | Update the server (apt update + upgrade + autoremove + autoclean) |
 | `lxs tool root-ssh-login` | Enable or disable root login over SSH with a password |
+| `lxs tool welcome` | View, edit, or reset the SSH welcome message (MOTD) |
 
 ## Project structure
 
@@ -76,7 +77,8 @@ lxs/
     ├── harden.sh
     ├── root-password.sh
     ├── update-server.sh
-    └── root-ssh-login.sh
+    ├── root-ssh-login.sh
+    └── welcome-message.sh
 ```
 
 After `lxs setup`, the full tree lives in `/usr/local/share/lxs/` and sub-scripts execute from disk. If `lxs.sh` is run without being installed (the one-liner mode), it falls back to downloading sub-scripts on demand.