feat: zsh default shell + starship/bat/eza/zoxide shell experience

zsh is now the default login shell (programs.zsh.enable +
users.defaultUserShell, mkOverride 500 to beat bash.nix's mkDefault;
per-user shell= still overrides). modules/home/shell.nix configures the
interactive experience (nomarchy.shell.enable):

- zsh: completion, autosuggestions, syntax highlighting, big shared
  history, autocd.
- starship prompt themed from the palette (a `nomarchy` palette built
  from theme-state.json colors).
- bat as cat (theme "ansi" → tracks the terminal/theme palette), eza as
  ls (icons + git + dirs-first), zoxide as cd (--cmd cd).
- rg/fd shipped as themselves — deliberately NOT aliased over grep/find
  (flag differences surprise).

home.shell.enableZshIntegration = true is the single lever; starship,
eza, zoxide, ghostty and yazi all emit their zsh hooks from it (no
per-module edits). Verified: flake check green, default shell resolves
to zsh, .zshrc carries the starship/zoxide/eza/yazi hooks + aliases,
starship palette renders, all tools in the live ISO closure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/options.nix

@@ -68,6 +68,7 @@
     idle.enable = lib.mkEnableOption "hyprlock + hypridle (idle lock, display off, suspend)" // { default = true; };
     yazi.enable = lib.mkEnableOption "the yazi TUI file manager, themed with a curated plugin set" // { default = true; };
     osd.enable = lib.mkEnableOption "swayosd on-screen display for volume/brightness/mute" // { default = true; };
+    shell.enable = lib.mkEnableOption "the zsh shell experience (starship prompt, bat/eza/zoxide)" // { default = true; };
     ghostty.enable = lib.mkEnableOption "Nomarchy's Ghostty configuration" // { default = true; };
     btop.enable = lib.mkEnableOption "btop with the per-theme nomarchy theme" // { default = true; };
     stylix.enable = lib.mkEnableOption "Stylix theming for the long tail of apps (GTK, Qt, cursors)" // { default = true; };