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/programs.nix

@@ -32,19 +32,6 @@
     ];
   };
   programs.fish.enable = true;
-  # Login shell is bash (see configuration.nix), but interactive login
-  # sessions exec straight into nushell. Non-interactive SSH commands
-  # (e.g. the Android Mosh app bootstrap `command -v mosh-server`)
-  # stay in bash where POSIX shell semantics work.
-  programs.bash = {
-    enable = true;
-    interactiveShellInit = ''
-      if [[ $- == *i* ]] && shopt -q login_shell && [[ -z "''${M3_SHELL_TO_NU:-}" ]]; then
-        export M3_SHELL_TO_NU=1
-        exec ${pkgs.nushell}/bin/nu --login
-      fi
-    '';
-  };
   programs.thunar = {
     enable = true;
     plugins = with pkgs; [thunar-archive-plugin thunar-volman];