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>

.gitignore

@@ -33,5 +33,9 @@
 !.config/micro/bindings.json
 !.config/micro/colorschemes/
 !.config/micro/colorschemes/nord-tc.micro
+!.config/bat/
+!.config/bat/config
+!.config/bat/themes/
+!.config/bat/themes/Nord.tmTheme
 !LICENSE
 !README.md