diskId: add migration docs and a big fat warning

docs/mkdocs.yml

@@ -79,6 +79,7 @@ nav:
           - Migrate existing Flakes: guides/migrations/migration-guide.md
           - Migrate inventory Services: guides/migrations/migrate-inventory-services.md
           - Facts Vars Migration: guides/migrations/migration-facts-vars.md
+          - Disk id: guides/migrations/disk-id.md
       - macOS: guides/macos.md
   - Reference:
       - Overview: reference/index.md

docs/site/guides/migrations/disk-id.md

@@ -0,0 +1,98 @@
+# Migrate disko config from `clanModules.disk-id`
+
+If you previously bootstrapped a machine's disk using `clanModules.disk-id`, you should now migrate to a standalone, self-contained disko configuration. This ensures long-term stability and avoids reliance on dynamic values from Clan.
+
+If your `disko.nix` currently looks something like this:
+
+```nix title="disko.nix"
+{
+  lib,
+  clan-core,
+  config,
+  ...
+}:
+
+let
+  suffix = config.clan.core.vars.generators.disk-id.files.diskId.value;
+in
+{
+  imports = [
+    clan-core.clanModules.disk-id
+  ];
+
+  # DO NOT EDIT THIS FILE AFTER INSTALLATION of a machine
+  # Otherwise your system might not boot because of missing partitions / filesystems
+  boot.loader.grub.efiSupport = lib.mkDefault true;
+  boot.loader.grub.efiInstallAsRemovable = lib.mkDefault true;
+  disko.devices = {
+    disk = {
+      "main" = {
+        # suffix is to prevent disk name collisions
+        name = "main-" + suffix;
+        type = "disk";
+        # Set the following in flake.nix for each maschine:
+        # device = <uuid>;
+        content = {
+          # edlied
+        };
+      };
+    };
+  };
+}
+```
+
+## Step 1: Retrieve your `disk-id`
+
+Run the following command to retrieve the generated disk ID for your machine:
+
+```bash
+clan vars list <machineName>
+```
+
+Which should print the generated `disk-id/diskId` value in clear text
+You should see output like:
+
+```terminal-session
+disk-id/diskId: fcef30a749f8451d8f60c46e1ead726f
+# ...
+# elided
+```
+
+Copy this value — you'll need it in the next step.
+
+## ✍️ Step 2: Replace Dynamic Configuration with Static Values
+
+✅ Goal: Make your disko.nix file standalone.
+
+We are going to make three changes:
+
+- Remove `let in, imports, {lib,clan-core,config, ...}:` to isolate the file.
+- Replace `suffix` with the actual disk-id
+- Move `disko.devices.disk.main.device` from `flake.nix` or `configuration.nix` into this file.
+
+```{.nix title="disko.nix" hl_lines="7-9 11-14"}
+{
+  boot.loader.grub.efiSupport = lib.mkDefault true;
+  boot.loader.grub.efiInstallAsRemovable = lib.mkDefault true;
+  disko.devices = {
+    disk = {
+      "main" = {
+        #       ↓ Copy the disk-id into place
+        name = "main-fcef30a749f8451d8f60c46e1ead726f";
+        type = "disk";
+
+        # Some earlier guides had this line in a flake.nix
+        # disko.devices.disk.main.device = "/dev/disk/by-id/__CHANGE_ME__";
+        #        ↓ Copy the '/dev/disk/by-id' into here instead
+        device = "/dev/disk/by-id/nvme-eui.e8238fa6bf530001001b448b4aec2929";
+
+        # edlied;
+      };
+    };
+  };
+}
+```
+
+These steps are only needed for existing configurations that depend on the `diskId` module.
+
+For newer machines clan offers simple *disk templates* via its [templates cli](../../reference/cli/templates.md)