dev: NixOS 25.11 매뉴얼 로컬 사본 추가

nixpkgs nixos-25.11 브랜치의 nixos/doc/manual/ 디렉토리를 sparse clone으로
가져와 ~/obsidian/dev/nixos-manual/에 복사. _index.md에 구조/갱신 방법 정리.

오프라인 참조 + AI 에이전트 컨텍스트용. sandbox-tokyo 같은 NixOS 노드 운영 시
빠른 참조로 사용.

dev/nixos-manual/administration/system-state.chapter.md

@@ -0,0 +1,17 @@
+# Necessary system state {#ch-system-state}
+
+Normally — on systems with a persistent `rootfs` — system services can persist state to
+the filesystem without administrator intervention.
+
+However, it is possible and not-uncommon to create [impermanent systems], whose
+`rootfs` is either a `tmpfs` or reset during boot. While NixOS itself supports
+this kind of configuration, special care needs to be taken.
+
+[impermanent systems]: https://wiki.nixos.org/wiki/Impermanence
+
+
+```{=include=} sections
+nixos-state.section.md
+systemd-state.section.md
+zfs-state.section.md
+```