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>

README.md

@@ -61,7 +61,8 @@ Flat on purpose. Two module trees, one options file each, no hidden layers.
 │       ├── swaync.nix             #   notifications, same JSON
 │       ├── idle.nix               #   hyprlock + hypridle, same JSON
 │       ├── yazi.nix               #   flagship TUI file manager + plugins
-│       └── osd.nix                #   swayosd volume/brightness OSD
+│       ├── osd.nix                #   swayosd volume/brightness OSD
+│       └── shell.nix              #   zsh + starship + bat/eza/zoxide
 ├── hosts/
 │   ├── default/                   # reference machine (thin: boot, user, hostname)
 │   └── live.nix                   # bootable live ISO (try the distro, no install)
@@ -178,6 +179,7 @@ examples: **[docs/OVERRIDES.md](docs/OVERRIDES.md)**.
 | `nomarchy.idle.enable` | `true` | hyprlock + hypridle (idle lock 5 min, display off 10, suspend 30) |
 | `nomarchy.yazi.enable` | `true` | yazi TUI file manager, themed + curated plugins |
 | `nomarchy.osd.enable` | `true` | swayosd on-screen display for volume/brightness/mute |
+| `nomarchy.shell.enable` | `true` | zsh + starship prompt + bat/eza/zoxide (zsh is the default login shell) |
 | `nomarchy.ghostty.enable` | `true` | Nomarchy's Ghostty |
 | `nomarchy.btop.enable` | `true` | btop with per-theme colors |
 | `nomarchy.stylix.enable` | `true` | GTK/Qt/cursor theming |
@@ -322,10 +324,12 @@ close · `SUPER+1..9` workspaces · `Print` region screenshot.
   Papirus-Dark/Light by `mode`) and feeds both Stylix's `gtk.iconTheme`
   (Thunar/GTK apps) and rofi's `show-icons`. Remaining (optional): per-theme
   `icons` overrides for the presets, or shipping more icon packs.
-- **Nicer shell out of the box:** ship and default-configure a prompt
-  (starship, themed from the JSON) plus modern-CLI ergonomics —
-  `bat`→cat, `eza`→ls, `ripgrep`→grep, `fd`→find, `zoxide`→cd — wired as
-  aliases/`$PAGER` so the terminal is pleasant by default (overridable).
+- **Nicer shell out of the box:** ✓ zsh is the default login shell, with a
+  starship prompt themed from the JSON, autosuggestions + syntax
+  highlighting, and modern-CLI ergonomics — `cat`→bat (theme "ansi", so it
+  tracks the palette), `ls`→eza, `cd`→zoxide, plus `lt`/`tree`.
+  `nomarchy.shell.enable`. (Deliberately did NOT alias grep→rg / find→fd:
+  their flags differ enough to surprise; `rg`/`fd` ship as themselves.)
 - **Default application suite:** install a complete-workstation set
   (vscode, libreoffice, gimp, inkscape, texlive-full, …) behind a
   `nomarchy.apps.*` option surface so each is individually opt-out for