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/shells/oh-my-posh.nix

@@ -7,7 +7,9 @@
     in
     {
       options.chiasson.home.shell.ohMyPosh = {
-        enable = lib.mkEnableOption "[Oh My Posh](https://ohmyposh.dev/) prompt.";
+        enable = lib.mkEnableOption "[Oh My Posh](https://ohmyposh.dev/) prompt." // {
+          default = true;
+        };
 
         builtinTheme = lib.mkOption {
           type = lib.types.str;