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/default.nix

@@ -3,7 +3,7 @@
   # Root module: chiasson.home.enable + bash. Other `wisdom*` slices auto-wire via
   # `lib.wisdomCatalogExtraModules`; hosts flip `chiasson.home.*.enable` rather than re-importing.
   flake.homeManagerModules.wisdom =
-    { config, lib, ... }:
+    { config, lib, pkgs, ... }:
     let
       cfg = config.chiasson.home;
     in
@@ -29,6 +29,20 @@
         };
       };
 
-      config = lib.mkIf cfg.enable { home.packages = cfg.extraPackages; };
+      config = lib.mkIf cfg.enable (lib.mkMerge [
+        { home.packages = cfg.extraPackages; }
+
+        # Base git identity + encryption/player tools shared by every HM user.
+        {
+          programs.git = {
+            enable = true;
+            settings.user = {
+              name = "OlivierChiasson";
+              email = "olivierchiasson@hotmail.fr";
+            };
+          };
+          home.packages = [ pkgs.git-crypt pkgs.feishin ];
+        }
+      ]);
     };
 }