fix(fish): update tmux session handling in shellInit

- Change default tmux session name from "default" to "0" for consistency
- Simplify session creation logic by removing explicit session naming

Author: takeokunn

home-manager/programs/fish/default.nix

@@ -20,7 +20,7 @@
     shellInit = ''
       # for tmux
       if type -q tmux && test -z $TMUX && status --is-login
-          tmux attach-session -t default || tmux new-session -s default
+          tmux attach-session -t 0 || tmux new-session
       end
 
       # suppress fish_greeting