bd init: initialize beads issue tracking

sites/default.nix

@@ -0,0 +1,20 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  cfg = config.az.tc;
+in {
+  imports = [./staging.nix ./production.nix];
+
+  options.az.tc.site = lib.mkOption {
+    type = lib.types.enum ["staging" "production"];
+    default = "staging";
+    description = "Site profile providing fleet-wide infrastructure defaults.";
+  };
+
+  config.assertions = lib.optional cfg.enable {
+    assertion = builtins.elem cfg.site ["staging" "production"];
+    message = "az.tc.site must be staging or production.";
+  };
+}