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/declarative-packages.section.md

@@ -0,0 +1,46 @@
+# Declarative Package Management {#sec-declarative-package-mgmt}
+
+With declarative package management, you specify which packages you want
+on your system by setting the option
+[](#opt-environment.systemPackages). For instance, adding the
+following line to `configuration.nix` enables the Mozilla Thunderbird
+email application:
+
+```nix
+{ environment.systemPackages = [ pkgs.thunderbird ]; }
+```
+
+The effect of this specification is that the Thunderbird package from
+Nixpkgs will be built or downloaded as part of the system when you run
+`nixos-rebuild switch`.
+
+::: {.note}
+Some packages require additional global configuration such as D-Bus or
+systemd service registration so adding them to
+[](#opt-environment.systemPackages) might not be sufficient. You are
+advised to check the [list of options](#ch-options) whether a NixOS
+module for the package does not exist.
+:::
+
+You can get a list of the available packages as follows:
+
+```ShellSession
+$ nix-env -qaP '*' --description
+nixos.firefox   firefox-23.0   Mozilla Firefox - the browser, reloaded
+...
+```
+
+The first column in the output is the *attribute name*, such as
+`nixos.thunderbird`.
+
+Note: the `nixos` prefix tells us that we want to get the package from
+the `nixos` channel and works only in CLI tools. In declarative
+configuration, use `pkgs` prefix (variable).
+
+To "uninstall" a package, remove it from
+[](#opt-environment.systemPackages) and run `nixos-rebuild switch`.
+
+```{=include=} sections
+customizing-packages.section.md
+adding-custom-packages.section.md
+```