WIP: Add theming system with /theme command

- Consolidated theme system into single src/theme/ directory
- Created Theme class with fg(), bg(), bold(), italic(), underline()
- Added dark and light built-in themes with 36 color tokens
- Support for custom themes in ~/.pi/agent/themes/*.json
- JSON schema for theme validation
- Theme selector UI with /theme command
- Save theme preference to settings
- Uses chalk for text formatting to preserve colors

TODO:
- Replace hardcoded colors throughout TUI components
- Apply markdown theming to Markdown components
- Add theme support to all TUI elements

packages/tui/src/index.ts

@@ -11,7 +11,7 @@ export {
 export { Editor, type TextEditorConfig } from "./components/editor.js";
 export { Input } from "./components/input.js";
 export { Loader } from "./components/loader.js";
-export { Markdown } from "./components/markdown.js";
+export { type DefaultTextStyle, Markdown, type MarkdownTheme } from "./components/markdown.js";
 export { type SelectItem, SelectList } from "./components/select-list.js";
 export { Spacer } from "./components/spacer.js";
 export { Text } from "./components/text.js";