refactor: archive old features directory to features.old

The new profile-based structure (home/base, home/desktop, home/server,
home/profiles/, home/coding) is fully operational and imported via
home/lib/mkHomeConfig. The legacy home/features directory is no longer
referenced anywhere in the configuration.

Archived rather than deleted to preserve history for reference.

home/features.old/desktop/obsidian.nix

@@ -0,0 +1,25 @@
+{
+  config,
+  lib,
+  ...
+}:
+with lib; let
+  cfg = config.features.desktop.obsidian;
+in {
+  options.features.desktop.obsidian.enable =
+    mkEnableOption "enable Obsidian knowledge base";
+
+  config = mkIf cfg.enable {
+    programs.obsidian.enable = true;
+
+    xdg.mimeApps = {
+      enable = true;
+      associations.added = {
+        "text/markdown" = ["obsidian.desktop"];
+      };
+      defaultApplications = {
+        "text/markdown" = ["obsidian.desktop"];
+      };
+    };
+  };
+}