cursa: build sd-card image with secret injection

`clan machines build cursa --format sd-card` failed in clan-cli's
post-build secret injection:

  Expected directory /nix/store/...-nixos-image-sd-card-...img.zst/sd-card not found in build output

clan-cli assumed $out/<format>/ (only true for iso; sd-card writes
$out/sd-image/) and tried to inject into every format although only iso
has an addFilesScript. Fixed upstream in
patches/upstream-sd-card-secret-injection.patch (pending PR); until it
lands:

- shell.nix: apply patches/clan-cli-image-filepath.patch (the clan-cli
  part of the upstream change) to the devshell's clan-cli.
- modules/nixos/sd-card-image.nix: local copy of the upstream
  clan.core.image.sd-card.addFilesScript module. Writes the vars tree
  directly into the ext4 root partition with debugfs (no boot-time
  hook), verifies with cmp + e2fsck. Delete once clan-core ships it.
- cursa imports it and builds its sd-card image without disko.

machines/cursa/configuration.nix

@@ -1,7 +1,12 @@
 {
+  self,
   ...
 }:
 {
+  imports = [
+    self.nixosModules.sd-card-image
+  ];
+
   clan.core.settings.machine.description =
     "VM machine for collecting prometheus metrics and fire alerts";
 
@@ -11,4 +16,10 @@
 
   system.stateVersion = "25.11";
   clan.core.sops.defaultGroups = [ "admins" ];
+
+  image.modules.sd-card = {
+    disabledModules = [
+      ./disko.nix
+    ];
+  };
 }