Render README preview directly under README header on repo root view

[rg4444]

Motivation

• Improve the repository default (root) file-list view UX by showing the rendered README preview immediately below the README header (the title + edit button) instead of inside the generic table flow.
• Reuse existing markup rendering styles so README preview appearance matches other rendered markup files.

Description

• Added a conditional block in templates/repo/view_file.tmpl that, when .ReadmeInList and .IsMarkup are true, outputs the rendered README HTML directly below the header using the existing file-view markup {{.MarkupType}} wrapper and {{.FileContent}} for content.
• Preserved the original table/segment rendering for all other cases by keeping the previous content in the else branch so non-README or non-markup behavior is unchanged.
• Change touches only the repository file view template to alter placement of the README preview in the root listing.

Testing

• Ran go test ./modules/markup/... and the markup-related tests completed successfully.
• Attempted a Playwright-based visual screenshot to validate placement but the headless Chromium process crashed in this environment, so no screenshot artifact was produced.

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Codex Task