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/rollback.section.md

@@ -0,0 +1,38 @@
+# Rolling Back Configuration Changes {#sec-rollback}
+
+After running `nixos-rebuild` to switch to a new configuration, you may
+find that the new configuration doesn't work very well. In that case,
+there are several ways to return to a previous configuration.
+
+First, the GRUB boot manager allows you to boot into any previous
+configuration that hasn't been garbage-collected. These configurations
+can be found under the GRUB submenu "NixOS - All configurations". This
+is especially useful if the new configuration fails to boot. After the
+system has booted, you can make the selected configuration the default
+for subsequent boots:
+
+```ShellSession
+# /run/current-system/bin/switch-to-configuration boot
+```
+
+Second, you can switch to the previous configuration in a running
+system:
+
+```ShellSession
+# nixos-rebuild switch --rollback
+```
+
+This is equivalent to running:
+
+```ShellSession
+# /nix/var/nix/profiles/system-N-link/bin/switch-to-configuration switch
+```
+
+where `N` is the number of the NixOS system configuration. To get a
+list of the available configurations, do:
+
+```ShellSession
+$ ls -l /nix/var/nix/profiles/system-*-link
+...
+lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
+```