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>

modules/environments/home-assistant/default.nix

@@ -1,8 +1,9 @@
-# manages home automations 
+# manages home automations
 {
   config,
   lib,
   pkgs,
+  self,
   ...
 }:
 let
@@ -10,6 +11,12 @@ let
   hostName = config.networking.hostName;
 in
 {
+  imports = [
+    # services.openthread-border-router isn't in nixos-25.11; pull from
+    # nixpkgs-unstable. Package comes from the existing unstable overlay.
+    "${self.inputs.nixpkgs-unstable}/nixos/modules/services/home-automation/openthread-border-router.nix"
+  ];
+
   options.my.profiles.home-assistant = with lib; {
     enable = mkEnableOption "Home Automation";