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

AGENTS.md

@@ -10,4 +10,5 @@
 - Always run `npm run check` in the project's root directory after making code changes.
 - You must NEVER run `npm run dev` yourself. Doing is means you failed the user hard.
 - Do NOT commit unless asked to by the user
-- Keep you answers short and concise and to the point.
+- Keep you answers short and concise and to the point.
+- Do NOT use inline imports ala `await import("./theme/theme.js");`