Codex Mobile: add a rich Markdown file viewer for project files

What variant of Codex are you using?

Codex Mobile / ChatGPT mobile app connected to a Codex host.

What feature would you like to see?

Codex Mobile should have a rich, mobile-optimized Markdown viewer for .md files opened from a connected Codex project/thread.

Today, Markdown project files are much harder to read on mobile when shown as raw source text. For files like README.md, specs, plans, generated reports, architecture notes, and issue drafts, mobile users need a rendered reading view similar to the Codex desktop Markdown preview.

Requested behavior:

• Opening a .md file from Codex Mobile should default to rendered Markdown.
• Users should be able to switch between rendered view and raw source view.
• The rendered view should handle headings, lists, tables, code blocks, links, images, task lists, and GitHub-style Markdown reasonably well.
• Relative links should resolve relative to the Markdown file when possible.
• The view should be comfortable on phone screens: readable text sizing, horizontal handling for wide tables/code blocks, and easy navigation through long docs.
• Copy/share actions should preserve either rendered text or Markdown source where appropriate.

Why this matters:

Codex Mobile is useful for supervising and steering long-running work away from a desktop, but many coding workflows require reading project documentation and generated Markdown artifacts. Raw Markdown is tolerable on desktop and painful on a phone. A rich Markdown viewer would make mobile review of specs, plans, READMEs, reports, and implementation notes much more practical.

This is related to #23631, which asks for remote Codex projects to render Markdown/HTML like local projects. This request is specifically for the Codex Mobile experience and the mobile file-viewing UX.