[codex] Integrate MarkdownRenderer in JitHub

[Xueyang-Song]

Summary

• replace JitHub markdown surfaces with the in-repo MarkdownRenderer wrapper using GFM, TextMate highlighting, app theming, code copy, selection, links, and image resolution
• add authenticated/private GitHub markdown image resolution and renderer image resolver plumbing
• restore and polish incremental loading for activities, issues, PRs, and commits with subtle list indicators
• fix search box focus behavior with app-wide Ctrl+K, Esc dismissal, keycap badge, focused ring, and UI automation coverage

Why

The old MarkdownTextBlock path could not support the production renderer behavior consistently across previews, comments, and forms. The integration also surfaced regressions around theme colors, layout, SVG badges, code-copy controls, incremental loading, and titlebar search focus that this PR closes out together.

Validation

• dotnet build JitHub.WinUI\JitHub.WinUI.csproj -c Debug -p:Platform=x64 --no-restore
• dotnet test JitHub.WinUI.Tests\JitHub.WinUI.Tests.csproj -c Debug -p:Platform=x64 --no-restore
• dotnet test MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -c Debug -p:Platform=x64 --no-restore
• dotnet run --project JitHub.WinUI.Automation\JitHub.WinUI.Automation.csproj -c Debug -- --probe=search-focus-contract --out=artifacts\screenshots\winui-search-focus
• git diff --check

Note: the WinUI build still reports the existing NETSDK1198 publish-profile warning for win-x64.pubxml; no build errors.