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/sources.chapter.md

@@ -0,0 +1,77 @@
+# Getting the Sources {#sec-getting-sources}
+
+By default, NixOS's `nixos-rebuild` command uses the NixOS and Nixpkgs
+sources provided by the `nixos` channel (kept in
+`/nix/var/nix/profiles/per-user/root/channels/nixos`). To modify NixOS,
+however, you should check out the latest sources from Git. This is as
+follows:
+
+```ShellSession
+$ git clone https://github.com/NixOS/nixpkgs
+$ cd nixpkgs
+$ git remote update origin
+```
+
+This will check out the latest Nixpkgs sources to `./nixpkgs` the NixOS
+sources to `./nixpkgs/nixos`. (The NixOS source tree lives in a
+subdirectory of the Nixpkgs repository.) The `nixpkgs` repository has
+branches that correspond to each Nixpkgs/NixOS channel (see
+[](#sec-upgrading) for more information about channels). Thus, the
+Git branch `origin/nixos-17.03` will contain the latest built and tested
+version available in the `nixos-17.03` channel.
+
+It's often inconvenient to develop directly on the master branch, since
+if somebody has just committed (say) a change to GCC, then the binary
+cache may not have caught up yet and you'll have to rebuild everything
+from source. So you may want to create a local branch based on your
+current NixOS version:
+
+```ShellSession
+$ nixos-version
+17.09pre104379.6e0b727 (Hummingbird)
+
+$ git checkout -b local 6e0b727
+```
+
+Or, to base your local branch on the latest version available in a NixOS
+channel:
+
+```ShellSession
+$ git remote update origin
+$ git checkout -b local origin/nixos-17.03
+```
+
+(Replace `nixos-17.03` with the name of the channel you want to use.)
+You can use `git merge` or `git
+  rebase` to keep your local branch in sync with the channel, e.g.
+
+```ShellSession
+$ git remote update origin
+$ git merge origin/nixos-17.03
+```
+
+You can use `git cherry-pick` to copy commits from your local branch to
+the upstream branch.
+
+If you want to rebuild your system using your (modified) sources, you
+need to tell `nixos-rebuild` about them using the `-I` flag:
+
+```ShellSession
+# nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs
+```
+
+If you want `nix-env` to use the expressions in `/my/sources`, use
+`nix-env -f
+  /my/sources/nixpkgs`, or change the default by adding a symlink in
+`~/.nix-defexpr`:
+
+```ShellSession
+$ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs
+```
+
+You may want to delete the symlink `~/.nix-defexpr/channels_root` to
+prevent root's NixOS channel from clashing with your own tree (this may
+break the command-not-found utility though). If you want to go back to
+the default state, you may just remove the `~/.nix-defexpr` directory
+completely, log out and log in again and it should have been recreated
+with a link to the root channels.