refactor: convert opentui from submodule to git dependency

- Remove packages/opentui submodule
- Add @opentui/core and @opentui/react as git dependencies from CodebuffAI/opentui#48803504
- Remove submodule-related scripts (postinstall, build:opentui)
- Remove packages/opentui/packages/* from workspaces
- Add required dependencies: yoga-layout, react-reconciler@^0.32.0, @opentui/core-darwin-arm64
- Delete .gitmodules file

Tested: CLI successfully runs with new git dependencies

Author: brandonkachen