Add ratatui, crossterm, and tui-textarea as optional dependencies behind
the `tui` feature flag. Create the `src/tui/` module with ActiveTab enum,
TuiApp struct with event loop, and placeholder tab renderers. Wire up
the `zeroclaw tui` CLI subcommand (cfg-gated) in main.rs.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
