Crossterm

The rat wires up the ears. We implement mode-specific event handlers with crossterm, redesign the key mapping to use a toggle instead of separate keys, solve the terminal restore problem with a capture-cleanup-return pattern, and run cargo test to discover that zero lines changed outside the adapter.

We start a new series by migrating the CLI adapter to a ratatui TUI. We set up the new dependencies, design the module structure under adapters/tui/, model the interaction modes as an enum to make invalid states unrepresentable, and solve the ownership puzzle of cloning a repository in a persistent session.