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/configuration/file-systems.chapter.md

@@ -0,0 +1,45 @@
+# File Systems {#ch-file-systems}
+
+You can define file systems using the `fileSystems` configuration
+option. For instance, the following definition causes NixOS to mount the
+Ext4 file system on device `/dev/disk/by-label/data` onto the mount
+point `/data`:
+
+```nix
+{
+  fileSystems."/data" = {
+    device = "/dev/disk/by-label/data";
+    fsType = "ext4";
+  };
+}
+```
+
+This will create an entry in `/etc/fstab`, which will generate a
+corresponding [systemd.mount](https://www.freedesktop.org/software/systemd/man/systemd.mount.html)
+unit via [systemd-fstab-generator](https://www.freedesktop.org/software/systemd/man/systemd-fstab-generator.html).
+The filesystem will be mounted automatically unless `"noauto"` is
+present in [options](#opt-fileSystems._name_.options). `"noauto"`
+filesystems can be mounted explicitly using `systemctl` e.g.
+`systemctl start data.mount`. Mount points are created automatically if they don't
+already exist. For `device`, it's best to use the topology-independent
+device aliases in `/dev/disk/by-label` and `/dev/disk/by-uuid`, as these
+don't change if the topology changes (e.g. if a disk is moved to another
+IDE controller).
+
+You can usually omit the file system type (`fsType`), since `mount` can
+usually detect the type and load the necessary kernel module
+automatically. However, if the file system is needed at early boot (in
+the initial ramdisk) and is not `ext2`, `ext3` or `ext4`, then it's best
+to specify `fsType` to ensure that the kernel module is available.
+
+::: {.note}
+System startup will fail if any of the filesystems fails to mount,
+dropping you to the emergency shell. You can make a mount asynchronous
+and non-critical by adding `options = [ "nofail" ];`.
+:::
+
+```{=include=} sections
+luks-file-systems.section.md
+sshfs-file-systems.section.md
+overlayfs.section.md
+```