Add visual dividers between conversation turns

When reading Claude Code output in the terminal, it's hard to visually separate where one turn ends and another begins. Other tools like Codex use horizontal dividing lines between turns, which makes the output much easier to scan.

Request: add a user-level setting (e.g. showTurnDividers) that renders a horizontal line between conversation turns. This could be a simple terminal-width line of dashes or box-drawing characters.

The infrastructure for this seems straightforward since turn boundaries are already tracked for showTurnDuration.