feat: add CLI interface with subcommands (#1398)

Replace environment variable-based configuration with a proper CLI
interface using subcommands and flags.

### Commands

```
openvpn-install <command> [options]

Commands:
  install       Install and configure OpenVPN server
  uninstall     Remove OpenVPN server
  client        Manage client certificates (add/list/revoke/renew)
  server        Server management (status/renew)
  interactive   Launch interactive menu
```

### Highlights

- **No args → help**: Running without arguments shows help instead of
interactive menu
- **JSON output**: `client list` and `server status` support `--format
json`
- **25+ install flags**: Network, DNS, security, and client options
- **Interactive mode preserved**: `install --interactive` or
`interactive` command

### Breaking Changes

Environment variables (`AUTO_INSTALL`, `MENU_OPTION`, `CLIENT`, etc.)
are no longer supported. Use CLI flags instead.

```bash
# Before
MENU_OPTION=1 CLIENT=foo PASS=1 ./openvpn-install.sh

# After
./openvpn-install.sh client add foo
```


Closes https://github.com/angristan/openvpn-install/issues/1202

.github/workflows/do-test.yml

@@ -97,7 +97,7 @@ jobs:
           host: ${{ steps.server_ip.outputs.value }}
           username: root
           key: ${{ secrets.SSH_KEY }}
-          script: 'set -x && AUTO_INSTALL=y bash -x ~/openvpn-install/openvpn-install.sh && ps aux | grep openvpn | grep -v grep > /dev/null 2>&1 && echo "Success: OpenVPN is running" && exit 0 || echo "Failure: OpenVPN is not running" && exit 1'
+          script: 'set -x && bash -x ~/openvpn-install/openvpn-install.sh install && ps aux | grep openvpn | grep -v grep > /dev/null 2>&1 && echo "Success: OpenVPN is running" && exit 0 || echo "Failure: OpenVPN is not running" && exit 1'
 
       - name: Delete server
         run: doctl compute droplet delete -f "openvpn-action-${GITHUB_RUN_ID}-${GITHUB_RUN_NUMBER}-${{ matrix.os-image }}"