Switch default shell from zsh to fish with starship prompt

Replace programs.zsh (powerlevel10k theme) with programs.fish and
starship prompt integration. Fish provides autosuggestions and syntax
highlighting out of the box, eliminating the need for separate plugins.

https://claude.ai/code/session_01NS2hpapno51a1Ng7hWaQKy

home.nix

@@ -1,22 +1,21 @@
-{ pkgs, ... }:
+{ ... }:
 
 {
   home.username = "chexit";
   home.homeDirectory = "/home/chexit";
   home.stateVersion = "25.11";
 
-  programs.zsh = {
+  programs.fish = {
     enable = true;
-    enableCompletion = true;
-    autosuggestion.enable = true;
-    syntaxHighlighting.enable = true;
-    initContent = ''
-      source ${pkgs.zsh-powerlevel10k}/share/zsh-powerlevel10k/powerlevel10k.zsh-theme
-    '';
     shellAliases = {
       rebuild = "sudo nixos-rebuild switch --flake ~/nixos-config#chexit";
     };
   };
 
+  programs.starship = {
+    enable = true;
+    enableFishIntegration = true;
+  };
+
   programs.home-manager.enable = true;
 }