feat(home-assistant): enable OTBR for ZBT-2 + add HA otbr/thread components

Brings up otbr-agent against the ZBT-2 over Spinel/UART, opens the
REST API on :8081, and wires HA's otbr + thread integrations so
Matter-over-Thread devices can commission through the existing
matter-server.

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

modules/environments/home-assistant/default.nix

@@ -31,6 +31,8 @@ in
       extraComponents = [
         "matter"
         "mobile_app"
+        "otbr"
+        "thread"
       ];
     };
     services.home-assistant.config = {
@@ -39,6 +41,14 @@ in
       mobile_app = {};
     };
 
+    services.openthread-border-router = {
+      enable = true;
+      package = pkgs.unstable.openthread-border-router;
+      openFirewall = true;
+      backboneInterfaces = [ "enp3s0" ];
+      radio.device = "/dev/serial/by-id/usb-Nabu_Casa_ZBT-2_DCB4D9149C7C-if00";
+    };
+
     my.homepage.services = [
       {
         group = "Services";