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/package-mgmt.chapter.md

@@ -0,0 +1,18 @@
+# Package Management {#sec-package-management}
+
+This section describes how to add additional packages to your system.
+NixOS has two distinct styles of package management:
+
+-   *Declarative*, where you declare what packages you want in your
+    `configuration.nix`. Every time you run `nixos-rebuild`, NixOS will
+    ensure that you get a consistent set of binaries corresponding to
+    your specification.
+
+-   *Ad hoc*, where you install, upgrade and uninstall packages via the
+    `nix-env` command. This style allows mixing packages from different
+    Nixpkgs versions. It's the only choice for non-root users.
+
+```{=include=} sections
+declarative-packages.section.md
+ad-hoc-packages.section.md
+```