[TUI] Render markdown for plan responses

What version of Codex CLI is running?

codex-cli 0.98.0

What subscription do you have?

ChatGPT Plus

Which model were you using?

gpt-5.3-codex

What platform is your computer?

Linux 6.17.0-14-generic x86_64 x86_64

What terminal emulator and version are you using (if applicable)?

GNOME Terminal 3.52.0 using VTE 0.76.0

What issue are you seeing?

Codex CLI shows plan responses as raw Markdown text instead of rendered formatting.

In my interactive terminal session, generated plan output displays literal Markdown tokens (e.g., ## headings and table pipe syntax) rather than rendered structure. This happens on a fresh Ubuntu install with latest Codex CLI.

What steps can reproduce the bug?

1. Install latest Codex CLI (@openai/codex) and run codex.
2. In an interactive terminal (TERM=xterm-256color, COLORTERM=truecolor, TTY=yes), ask:

give me a random plan just so i can check the markdown rendering

1. Observe the response in the TUI.
2. Markdown is shown literally (e.g., heading markers and table syntax) instead of rendered formatting.

What is the expected behavior?

Plan output appears formatted in the CLI exactly as it did before, with Markdown presentation rendered in the terminal view rather than displayed as raw Markdown source text.

Additional information

<img width="1497" height="1524" alt="Image" src="https://github.com/user-attachments/assets/06fc066c-c327-454b-8fe8-416ec48dc052" />