feat: comprehensive NixOS and home-manager configuration updates

This commit includes significant updates to the Nix configuration including:
- Added common NixOS module with system packages
- Updated home-manager configurations for various systems
- Enhanced terminal programs including Claude Code integration
- Updated LLM server configurations with new models
- Improved btop configuration with custom package
- Added opencode support with permission settings
- Updated tmux and nvim configurations

The changes introduce new features, update existing configurations, and improve the overall system setup.

homes/x86_64-linux/evanreichard@lin-va-desktop/default.nix

@@ -0,0 +1,30 @@
+{ lib
+, config
+, namespace
+, ...
+}:
+let
+  inherit (lib.${namespace}) enabled;
+in
+{
+  home.stateVersion = "25.11";
+
+  reichard = {
+    user = {
+      enable = true;
+      inherit (config.snowfallorg.user) name;
+    };
+
+    services = {
+      ssh-agent = enabled;
+    };
+
+    programs = {
+      terminal = {
+        bash = enabled;
+        btop = enabled;
+        tmux = enabled;
+      };
+    };
+  };
+}