fix(tui): correct delete keybind behavior

[slimewell]

Summary

• fix option-delete to avoid removing a newline when deleting a single-word line
• make cmd+backspace consistently delete to line start or join lines at boundaries
• add default super+backspace binding for delete-to-line-start

Testing

• bun dev (manual repro in TUI prompt)

Demo

• uploading video

Fixes #6314, adds #436