feat(agent): service systemd DynamicUser + documentation déploiement

- Ajoute deploy/nanometrics-agent.service avec DynamicUser, ProtectSystem,
  PrivateTmp, NoNewPrivileges et RestrictAddressFamilies
- Ajoute deploy/README.md avec procédure d'installation/désinstallation
- Corrige 3 avertissements clippy : iter_kv_map (network.rs) et
  collapsible_match (mqtt.rs)

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

deploy/nanometrics-agent.service

@@ -0,0 +1,24 @@
+[Unit]
+Description=Nanometrics Agent — collecte de métriques système
+After=network.target
+Documentation=https://git.maison43gil.com/gilles/nano_metrics
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/nanometrics-agent /etc/nanometrics/config.toml
+Restart=on-failure
+RestartSec=5
+
+DynamicUser=yes
+ConfigurationDirectory=nanometrics
+ConfigurationDirectoryMode=0750
+
+ProtectSystem=strict
+ProtectHome=read-only
+PrivateTmp=yes
+NoNewPrivileges=yes
+
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
+
+[Install]
+WantedBy=multi-user.target