add Linux systemd service and DNS configuration

Linux:
- numa install: backs up /etc/resolv.conf, sets nameserver to 127.0.0.1
- numa uninstall: restores original /etc/resolv.conf from backup
- numa service start: installs systemd unit, enables + starts
- numa service stop: stops, disables, removes unit file
- numa service status: shows systemctl status

macOS: launchd plist (already working)

Both platforms: Restart=always / KeepAlive=true for crash recovery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

numa.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Numa DNS — DNS you own, everywhere you go
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/numa
+Restart=always
+RestartSec=2
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=numa
+
+[Install]
+WantedBy=multi-user.target