feat: disko OS-disk layout + nixos-anywhere install flow

- add disko input; jupiter partitions/formats OS disk declaratively
- hardware-configuration.nix carries kernel modules only (disko owns fileSystems)
- data disk stays a plain unformatted mount, out of disko
- vbox unchanged (virtualbox-image supplies its own disk)
- README: nixos-anywhere remote install + daily rebuild loop

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

jupiter/disk-config.nix

@@ -0,0 +1,42 @@
+{ ... }:
+
+# Declarative OS-disk layout (disko). UEFI: GPT with an ESP + ext4 root.
+# disko both PARTITIONS/FORMATS this disk and generates the NixOS
+# `fileSystems.*` entries, so hardware-configuration.nix must NOT define
+# fileSystems for "/" or "/boot".
+#
+# ⚠️  This disk is WIPED on install. Set `device` to the OS disk ONLY.
+#     Your NAS data disk is NOT listed here — keep it out of disko so it is
+#     never formatted; mount it as a plain read-write fileSystem instead
+#     (see configuration.nix, /mnt/data).
+#
+# Find the stable id:  ls -l /dev/disk/by-id   (use by-id, never /dev/sdX)
+{
+  disko.devices.disk.os = {
+    type = "disk";
+    device = "/dev/disk/by-id/CHANGE-ME-os-disk";
+    content = {
+      type = "gpt";
+      partitions = {
+        ESP = {
+          size = "512M";
+          type = "EF00";
+          content = {
+            type = "filesystem";
+            format = "vfat";
+            mountpoint = "/boot";
+            mountOptions = [ "umask=0077" ];
+          };
+        };
+        root = {
+          size = "100%";
+          content = {
+            type = "filesystem";
+            format = "ext4";
+            mountpoint = "/";
+          };
+        };
+      };
+    };
+  };
+}