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/hosts/desktop-home-base.nix

@@ -17,10 +17,6 @@
         userCli.enable = true;
       };
 
-      chiasson.users.extraModules.olivier =
-        self.lib.wisdomCatalogExtraModules self
-        ++ [
-          self.homeManagerModules.desktopHomeDefaults
-        ];
+      chiasson.users.extraModules.olivier = self.lib.wisdomCatalogExtraModules self;
     };
 }