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/development/replace-modules.section.md

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+# Replace Modules {#sec-replace-modules}
+
+Modules that are imported can also be disabled. The option declarations,
+config implementation and the imports of a disabled module will be
+ignored, allowing another to take its place. This can be used to
+import a set of modules from another channel while keeping the rest of
+the system on a stable release.
+
+`disabledModules` is a top level attribute like `imports`, `options` and
+`config`. It contains a list of modules that will be disabled. This can
+either be:
+ - the full path to the module,
+ - or a string with the filename relative to the modules path (eg. \<nixpkgs/nixos/modules> for nixos),
+ - or an attribute set containing a specific `key` attribute.
+
+The latter allows some modules to be disabled, despite them being distributed
+via attributes instead of file paths. The `key` should be globally unique, so
+it is recommended to include a file path in it, or rely on a framework to do it
+for you.
+
+This example will replace the existing postgresql module with the
+version defined in the nixos-unstable channel while keeping the rest of
+the modules and packages from the original nixos channel. This only
+overrides the module definition, this won't use postgresql from
+nixos-unstable unless explicitly configured to do so.
+
+```nix
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  disabledModules = [ "services/databases/postgresql.nix" ];
+
+  imports = [
+    # Use postgresql service from nixos-unstable channel.
+    # sudo nix-channel --add https://channels.nixos.org/nixos-unstable nixos-unstable
+    <nixos-unstable/nixos/modules/services/databases/postgresql.nix>
+  ];
+
+  services.postgresql.enable = true;
+}
+```
+
+This example shows how to define a custom module as a replacement for an
+existing module. Importing this module will disable the original module
+without having to know its implementation details.
+
+```nix
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  inherit (lib) mkIf mkOption types;
+  cfg = config.programs.man;
+
+in
+{
+  disabledModules = [ "services/programs/man.nix" ];
+
+  options = {
+    programs.man.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Whether to enable manual pages.";
+    };
+  };
+
+  config = mkIf cfg.enabled {
+    warnings = [ "disabled manpages for production deployments." ];
+  };
+}
+```