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/common/default.nix

@@ -76,5 +76,14 @@
       ((lib.filterAttrs (_: lib.isType "flake")) inputs);
     nixPath = ["/etc/nix/path"];
   };
-  users.defaultUserShell = pkgs.nushell;
+  users.defaultUserShell = pkgs.bashInteractive;
+  programs.bash = {
+    enable = true;
+    interactiveShellInit = ''
+      if [[ $- == *i* ]] && [[ -z "''${M3_SHELL_TO_NU:-}" ]]; then
+        export M3_SHELL_TO_NU=1
+        exec ${pkgs.nushell}/bin/nu --login
+      fi
+    '';
+  };
 }

hosts/common/users/m3tam3re.nix

@@ -236,6 +236,9 @@ in {
 
   # ── Home-Manager configuration via m3ta-home ──
   home-manager.users.m3tam3re = {
+    home.file.".bashrc".text = ''
+      . /etc/bashrc
+    '';
     imports =
       [
         # Load m3ta-home composition engine