Ability to add comments in Markdown Preview Mode

Request

Please add support for leaving review comments while viewing a Markdown file in Preview Mode.

Reasoning

As AI-assisted workflows become more common, more of the important review context is showing up in Markdown files, not just code. Things like planning docs, specs, implementation notes, READMEs, and BMAD-style workflow docs are becoming a much bigger part of the review process.

Preview Mode is the best way to review that content because it shows the Markdown as it is actually intended to be read. But right now, if I want to leave a review comment, I have to switch back to the raw Markdown source. That breaks the flow and makes it harder to review and comment on symbol heavy components; such as tables, ascii diagrams, etc.

It would be really helpful to support comments directly from Preview Mode since it would make reviewing Markdown-heavy PRs much smoother and more natural.

Examples

BMad Method, where teams work through Markdown artifacts such as PRDs, architecture documents, story files, and user guides, and Superpowers, which uses Markdown specs and implementation plans as key workflow artifacts. In these kinds of frameworks, Markdown files are not just supporting docs; they are a primary medium for planning, review, and inter-agent coordination.

I am aware of this previous feature-request post, but it was made 2+ years ago and seems like it never went anywhere. I think the importance of this has changed drastically since then. - https://github.com/microsoft/vscode-pull-request-github/issues/5382

Start by reading the linked previous feature request, issue #5382, and tracing how Markdown Preview Mode and review comments are handled in this extension. Define the preview interaction and verify that reviewers can select Markdown content, leave comments, and retain the expected review context without switching to source view.