feat(tools): add root-password and update-server tools

- add `tools/root-password.sh` to change root password interactively or generate a random one
- add `tools/update-server.sh` to run apt update/upgrade/autoremove/autoclean
- register both tools in `lxs.sh` dispatcher and `tools/index.sh` interactive menu
- document new tools in README.md

README.md

@@ -50,6 +50,8 @@ lxs help                  # Show help
 | `lxs tool system` | System monitoring and diagnostics |
 | `lxs tool benchmark` | Server benchmark (CPU / RAM / disk / network) |
 | `lxs tool harden` | Baseline hardening: UFW + fail2ban + SSH key-only + unattended-upgrades |
+| `lxs tool root-password` | Change the root password (interactive or generated) |
+| `lxs tool update` | Update the server (apt update + upgrade + autoremove + autoclean) |
 
 ## Project structure
 
@@ -70,7 +72,9 @@ lxs/
     ├── index.sh           # Interactive menu listing the tools below
     ├── system-infos.sh
     ├── server-benchmark.sh
-    └── harden.sh
+    ├── harden.sh
+    ├── root-password.sh
+    └── update-server.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.