refactor(wisdom): decentralize module defaults and update conventions

Move Home Manager module defaults from a centralized `desktop-home-defaults.nix`
file into their respective `wisdom` slices. This change aligns with the
updated documentation regarding module ownership, where defaults live
with the option that owns them rather than in a shared defaults file.

Additionally, update `docs/conventions.md` to reflect this new pattern for
defining `mkEnableOption` defaults and move base git/encryption
configurations into the core `wisdom` module.

- Delete `modules/hosts/desktop-home-defaults.nix`
- Update `modules/wisdom/default.nix` to include shared git and package defaults
- Add `default = ...` to `mkEnableOption` in various `wisdom` modules
- Update `docs/conventions.md` with new HM slice guidelines

modules/wisdom/desktop/gtk-qt-theming.nix

@@ -12,7 +12,9 @@
         enable = lib.mkEnableOption ''
           WhiteSur GTK + icon themes, Phinger cursor, and Qt via qt6ct (DMS) or KDE platform theme.
           Matugen accent colors load via gtk3/gtk4 extraCss when matugenGtkColors is enabled.
-        '';
+        '' // {
+          default = true;
+        };
 
         matugenGtkColors = lib.mkOption {
           type = lib.types.bool;