refactor: make nanuqsaurus module-based; keep btrfs install profile

docs/adopt.md

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+# Adopt Nanuqsaurus on an existing NixOS install (no repartitioning)
+
+This repo intentionally does **not** ship a “one size fits all” `nixosConfiguration` for installed systems.
+Disk layouts, filesystems, initrd modules, and bootloader details are machine-specific and live in your
+local `hardware-configuration.nix`.
+
+The recommended workflow is to create a **small local wrapper flake** that:
+
+- keeps your machine’s `hardware-configuration.nix` as the source of truth for mounts/boot,
+- imports `nanuqsaurus` as a feature/profile module from its URL,
+- keeps your personal changes in your own repo (or `/etc/nixos`).
+
+## 1) Create a wrapper flake in `/etc/nixos`
+
+On the target machine:
+
+1. Ensure you have a `hardware-configuration.nix` (generated by `nixos-generate-config`).
+2. Create `/etc/nixos/flake.nix` like this (adjust `hostName` and `stateVersion`):
+
+```nix
+{
+  description = "Local wrapper for the nanuqsaurus profile";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    nanuqsaurus.url = "git+https://codeberg.org/randogoth/nanuqsaurus.git";
+    nanuqsaurus.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs =
+    { self, nixpkgs, nanuqsaurus, ... }:
+    {
+      nixosConfigurations.my-host = nixpkgs.lib.nixosSystem {
+        system = "x86_64-linux";
+        modules = [
+          ./hardware-configuration.nix
+          nanuqsaurus.nixosModules.nanuqsaurus
+
+          # Your local overrides/customizations:
+          ({ ... }: {
+            # Optional: change the default admin username ("admin")
+            nanuqsaurus.admin.username = "admin";
+            networking.hostName = "my-host";
+            system.stateVersion = "25.11";
+          })
+        ];
+      };
+    };
+}
+```
+
+## 2) Switch to it
+
+```bash
+sudo nixos-rebuild switch --flake /etc/nixos#my-host
+```
+
+## Updating later
+
+- If you keep `nanuqsaurus.url` pointing at the repo, you can update with:
+  - `nix flake update` (in `/etc/nixos`), then
+  - `sudo nixos-rebuild switch --flake /etc/nixos#my-host`
+
+- Keep all your machine-specific changes in the wrapper flake (additional modules, packages, services, etc.).