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/browsers/zen.nix

@@ -8,7 +8,9 @@
     {
       imports = [ inputs.zen-browser.homeModules.beta ];
 
-      options.chiasson.home.browsers.zen.enable = lib.mkEnableOption "Zen Browser + locked-down policies / extensions.";
+      options.chiasson.home.browsers.zen.enable = lib.mkEnableOption "Zen Browser + locked-down policies / extensions." // {
+        default = false;
+      };
 
       config = lib.mkIf (root.enable && cfg.enable) {
         programs.zen-browser = {