Switch from systemd-networkd to NetworkManager, add AGENTS.md

- Replace systemd-networkd/resolved symlinks with NetworkManager
- Remove unused networkd config files (ethernet, wlan, wwan)
- Add AGENTS.md for CI/agent guidance
- Update motd and packages.x86_64

airootfs/etc/systemd/system/multi-user.target.wants/NetworkManager.service

@@ -0,0 +1,30 @@
+[Unit]
+Description=Network Manager
+Documentation=man:NetworkManager.conf(5)
+Wants=NetworkManager-wait-online.service
+After=sysinit.target network-pre.target
+Wants=network-pre.target
+Before=network-online.target remote-fs.target
+After=network-pre.target
+BindsTo=sys-subsystem-net-devices.target
+After=sys-subsystem-net-devices.target
+
+[Service]
+Type=dbus
+BusName=org.freedesktop.NetworkManager
+ExecCondition=/usr/bin/test -x /usr/bin/NetworkManager
+ExecReload=/usr/bin/kill -HUP $MAINPID
+ExecStart=/usr/bin/NetworkManager --no-daemon
+Restart=on-failure
+KillMode=process
+CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH CAP_DAC_OVERRIDE
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+ProtectHome=yes
+ProtectSystem=yes
+RestrictNamespaces=yes
+RestrictRealtime=yes
+SystemCallFilter=@system-service
+
+[Install]
+WantedBy=multi-user.target