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/screenshot.nix

@@ -13,11 +13,13 @@
     {
       options.chiasson.home.desktop.screenshot.enable = lib.mkEnableOption ''
         grim/slurp/swappy + SwiftShare helpers; Hyprland binds if HM Hyprland is on.
-      '';
+      '' // {
+        default = true;
+      };
 
       options.chiasson.home.desktop.screenshot.swiftshareApiKeyFile = lib.mkOption {
         type = lib.types.nullOr lib.types.str;
-        default = null;
+        default = "/run/secrets/swiftshare/API_KEY";
         description = ''
           File with SwiftShare API key (sops path is fine). Required when screenshot module is on.
         '';