feat(home-assistant): import openthread-border-router module from unstable

Pulls the services.openthread-border-router NixOS module directly from
nixpkgs-unstable since it isn't in 25.11 yet. Service stays disabled
in this commit; configuration follows.

Also promotes `self` from `_module.args` to `specialArgs` in
machines/configuration.nix, since `imports` are evaluated before
`config` and so can't reach `_module.args.self`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

machines/configuration.nix

@@ -49,6 +49,7 @@ in
     # use your hardware- model from this list: https://github.com/NixOS/nixos-hardware/blob/master/flake.nix
     jupiter = nixosSystem {
       system = "x86_64-linux";
+      specialArgs = { inherit self; };
       modules = defaultModules ++ [
         # nixos-hardware.nixosModules.bmax-b7-power
         ./jupiter/configuration.nix
@@ -56,6 +57,7 @@ in
     };
     mibook = nixosSystem {
       system = "x86_64-linux";
+      specialArgs = { inherit self; };
       modules = defaultModules ++ [
         # nixos-hardware.nixosModules.mibook
         ./mibook/configuration.nix