Vim mode

This is mostly rustyline issue, but this issue tracks the status
* [x] `dd` and similar line commands should handle single line
* [ ] visual selection mode

The issue identifies rustyline and the CLI's Vim mode, with visual selection mode as the remaining task. Start by reviewing the existing line-command handling and rustyline integration; no file or test is named, so define completion around working visual selection behavior and add or update coverage where the implementation is found.

Written by the indexing model from the issue text.