[CLI] Always show a visual separator between conversation turns

What variant of Codex are you using?

CLI

What feature would you like to see?

Problem

In long Codex CLI conversations, user prompts are difficult to locate. They appear as:

› My question

followed immediately by assistant output:

• Response

A horizontal separator currently appears only when the preceding turn performed tool work. Purely conversational turns
receive no separator, making navigation inconsistent.

Proposed behavior

Always render a horizontal separator between a completed assistant turn and the next user prompt.

An opt-in setting would also solve the problem:

[tui]
always_show_turn_separator = true

## Acceptance criteria

- Every top-level user prompt has a clear visual boundary above it.
- Separators appear after conversational and tool-using turns.
- The behavior remains consistent after resuming a conversation.
- The Worked for … label can remain limited to longer work turns. Only the horizontal divider needs to be unconditional.

### Additional information

_No response_

No file or test is named in the issue. Start by locating the CLI/TUI rendering path that decides whether a turn separator is shown, then reproduce conversational and tool-using turns, including a resumed conversation; it is done when every top-level user prompt has a boundary while the Worked for … label remains conditional.