feat: add Nord theme for bat

- .config/bat/themes/Nord.tmTheme — full tmTheme covering comments,
  strings, numbers, keywords, functions, types, tags, CSS, markdown,
  diff markup, decorators, regex, YAML/TOML/JSON, shell
- .config/bat/config — sets Nord as default theme
- arty.yml: bat:build task to rebuild cache after cloning dotfiles
- .gitignore: whitelist bat config and theme

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arty.yml

@@ -74,3 +74,4 @@ scripts:
     mkdir -p ~/.local/share/fonts/Meslo
     curl -L https://github.com/ryanoasis/nerd-fonts/releases/latest/download/Meslo.tar.xz | tar --extract --xz --directory ~/.local/share/fonts/Meslo
     fc-cache -fv
+  bat:build: bat cache --build