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/installation/installing-behind-a-proxy.section.md

@@ -0,0 +1,31 @@
+# Installing behind a proxy {#sec-installing-behind-proxy}
+
+To install NixOS behind a proxy, do the following before running
+`nixos-install`.
+
+1.  Update proxy configuration in `/mnt/etc/nixos/configuration.nix` to
+    keep the internet accessible after reboot.
+
+    ```nix
+    {
+      networking.proxy.default = "http://user:password@proxy:port/";
+      networking.proxy.noProxy = "127.0.0.1,localhost,internal.domain";
+    }
+    ```
+
+1.  Setup the proxy environment variables in the shell where you are
+    running `nixos-install`.
+
+    ```ShellSession
+    # proxy_url="http://user:password@proxy:port/"
+    # export http_proxy="$proxy_url"
+    # export HTTP_PROXY="$proxy_url"
+    # export https_proxy="$proxy_url"
+    # export HTTPS_PROXY="$proxy_url"
+    ```
+
+::: {.note}
+If you are switching networks with different proxy configurations, use
+the `specialisation` option in `configuration.nix` to switch proxies at
+runtime. Refer to [](#ch-options) for more information.
+:::