fix(shell): exec into nushell by parent process, not env marker

The exported M3_SHELL_TO_NU marker leaked into programs started from
nu (zellij, zed, herdr), keeping their spawned terminals in bash.
Decide via /proc/$PPID/comm instead: interactive bash whose parent is
neither nu nor bash execs into nu; a deliberate 'bash' started from
nu/bash stays bash. Non-interactive bash (ssh command exec, Mosh app
bootstrap, see 765ae15) is still covered by the $- == *i* check.

hosts/common/default.nix

@@ -80,9 +80,17 @@
   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
+      # Interactive bash -> nushell, unless started deliberately from
+      # nu/bash (parent check — an exported env marker would leak into
+      # zellij/zed/herdr children). Non-interactive bash (ssh command
+      # exec, Mosh app bootstrap) stays bash.
+      if [[ $- == *i* ]]; then
+        parent=""
+        [[ -r /proc/$PPID/comm ]] && IFS= read -r parent < /proc/$PPID/comm
+        if [[ "$parent" != "nu" && "$parent" != "bash" ]]; then
+          exec ${pkgs.nushell}/bin/nu --login
+        fi
+        unset parent
       fi
     '';
   };