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
- Introduced a new module `rbw-ssh-socket.nix` to manage SSH_AUTH_SOCK for rbw.
- Updated `default.nix` to utilize the new SSH socket variable for session management.
- Refactored Bash and Fish shell configurations to use the new module for setting SSH_AUTH_SOCK, improving maintainability and consistency across shell environments.
Olivier