[FEATURE]: WYSIWYG Preview & Edit for docx / HTML / Markdown

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Problem:
When using opencode to generate or modify documents (docx, HTML, Markdown), the current workflow is entirely code-diff based — you see raw diffs of the source, not a rendered preview of the final document. This makes it difficult to:

• Judge whether the visual/layout result is correct before applying changes.
• Make fine-grained adjustments to rendered content (e.g., tweak a table layout in docx, adjust heading hierarchy in Markdown, or fix an HTML element's styling).
• Iterate quickly on document content without going through many /undo + re-prompt cycles.

Solution:
A preview/edit mode for rendered document types (docx, HTML, Markdown) that works like a WYSIWYG editor:

1. After opencode generates or modifies a document, show a rendered preview of the result (not just source diff).
2. Allow the user to:
   • Directly edit text, formatting, and layout within the rendered preview.
   • See changes reflected in real-time.
   • Confirm or discard individual edits before finalizing.
3. The underlying source (docx/HTML/MD) is updated accordingly after confirmation.