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/administration/control-groups.chapter.md

@@ -0,0 +1,59 @@
+# Control Groups {#sec-cgroups}
+
+To keep track of the processes in a running system, systemd uses
+*control groups* (cgroups). A control group is a set of processes used
+to allocate resources such as CPU, memory or I/O bandwidth. There can be
+multiple control group hierarchies, allowing each kind of resource to be
+managed independently.
+
+The command `systemd-cgls` lists all control groups in the `systemd`
+hierarchy, which is what systemd uses to keep track of the processes
+belonging to each service or user session:
+
+```ShellSession
+$ systemd-cgls
+├─user
+│ └─eelco
+│   └─c1
+│     ├─ 2567 -:0
+│     ├─ 2682 kdeinit4: kdeinit4 Running...
+│     ├─ ...
+│     └─10851 sh -c less -R
+└─system
+  ├─httpd.service
+  │ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
+  │ └─...
+  ├─dhcpcd.service
+  │ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
+  └─ ...
+```
+
+Similarly, `systemd-cgls cpu` shows the cgroups in the CPU hierarchy,
+which allows per-cgroup CPU scheduling priorities. By default, every
+systemd service gets its own CPU cgroup, while all user sessions are in
+the top-level CPU cgroup. This ensures, for instance, that a thousand
+run-away processes in the `httpd.service` cgroup cannot starve the CPU
+for one process in the `postgresql.service` cgroup. (By contrast, it
+they were in the same cgroup, then the PostgreSQL process would get
+1/1001 of the cgroup's CPU time.) You can limit a service's CPU share in
+`configuration.nix`:
+
+```nix
+{ systemd.services.httpd.serviceConfig.CPUShares = 512; }
+```
+
+By default, every cgroup has 1024 CPU shares, so this will halve the CPU
+allocation of the `httpd.service` cgroup.
+
+There also is a `memory` hierarchy that controls memory allocation
+limits; by default, all processes are in the top-level cgroup, so any
+service or session can exhaust all available memory. Per-cgroup memory
+limits can be specified in `configuration.nix`; for instance, to limit
+`httpd.service` to 512 MiB of RAM (excluding swap):
+
+```nix
+{ systemd.services.httpd.serviceConfig.MemoryLimit = "512M"; }
+```
+
+The command `systemd-cgtop` shows a continuously updated list of all
+cgroups with their CPU and memory usage.