Codex Desktop: support in-place Markdown editing in the side file viewer

Feature request

Codex Desktop can open and render local Markdown files in the right-side file viewer, but the current experience is effectively preview/review only. For document-heavy workflows, users often want to treat Markdown specs, PRDs, design notes, and planning docs as shared working documents inside Codex instead of constantly switching to an external editor.

I would like Codex to support a Claude-artifacts-like editing workflow for Markdown files opened in the side panel. Claude artifacts support selecting Markdown content and asking Claude to edit that region in place; Codex would benefit from a similar workflow adapted to local workspace files and disk-backed changes.

Desired behavior

• Open a local .md file in the Codex side file viewer and switch between rendered preview and source/edit mode.
• Allow direct Markdown source editing in the side panel, with clear dirty state, save, and revert behavior.
• In rendered Markdown view, allow selecting text/regions and submitting an edit request or comment to the active Codex thread.
• Preserve enough source context for Codex to map the selected rendered content back to Markdown line ranges where possible.
• When Codex patches the file or the file changes on disk, refresh the rendered view automatically or show a clear stale/out-of-date indicator with a refresh action.
• Preserve scroll position across edits, refreshes, and thread navigation where possible.
• Keep existing review safeguards: users should be able to inspect diffs and understand what changed on disk.

Why this matters

A lot of Codex work is not just code editing. Users also create and iterate on Markdown product specs, design notes, implementation plans, review summaries, and generated reports. Today, the side viewer is useful for inspection, but not enough for document collaboration.

The current workflow creates friction:

1. Open or generate a Markdown file in Codex.
2. Read it in the right-side rendered preview.
3. Notice a section that needs editing.
4. Either switch to an external editor, or describe the section manually in chat.
5. If Codex edits the file, the open rendered view may not refresh immediately.

This makes Codex feel less like a document workspace and more like a static preview pane. In-place Markdown editing and selection-based edit requests would make Codex much better for product/design/spec workflows while staying grounded in local files.

Related issues

This request overlaps with, but is broader than:

• #15399: Open files directly in Codex app / built-in viewer-editor target.
• #26198: Selection-based comments in Markdown and HTML previews.
• #24399: Live refresh for rendered Markdown documents.
• #18129: Sidebar file view does not auto-refresh when AI modifies open files.
• #22095: Preserve scroll position and support keyboard paging in the file viewer.

Acceptance criteria

• A user can edit a local Markdown file without leaving Codex.
• A user can select text in rendered Markdown and ask Codex to revise that selected section without manually restating its location.
• Codex preserves source/file context for the selected Markdown region.
• The side viewer reflects file changes after Codex edits the file.
• The user can still inspect disk changes through the normal diff/review flow.