refactor(shell): move bash->nushell wrapper to common

Default login shell is now bash for all hosts; interactive bash
sessions exec into nushell via /etc/bashrc. A ~/.bashrc sourcing
/etc/bashrc covers non-login interactive shells (terminal emulators).
Non-interactive SSH exec (mosh app bootstrap, scripts) keeps POSIX
shell semantics. Reverts the kratos-host-specific overlay from
765ae15.

hosts/m3-kratos/configuration.nix

@@ -70,12 +70,6 @@
 
   # Define a user account. Don't forget to set a password with ‘passwd’.
 
-  # bash as login shell wrapper: non-interactive SSH clients (e.g. the
-  # Android Mosh app bootstrap, which runs `command -v mosh-server`)
-  # need POSIX shell semantics. Interactive login sessions exec into
-  # nushell automatically — see programs.bash in ./programs.nix.
-  users.users.m3tam3re.shell = pkgs.bashInteractive;
-
   # List packages installed in system profile. To search, run:
   # $ nix search wget
   environment.systemPackages = with pkgs; [neovim git];