[codex] Fix README screenshot asset URLs

[premsathisha]

Summary

• fix repo README rendering so relative image assets resolve against GitHub raw content URLs
• support both markdown image src and raw HTML source srcset assets used by screenshots
• add focused coverage for root, root-relative, nested, absolute, and anchor URLs

Why

README screenshots were broken for repos that reference checked-in assets with relative paths, including the examples from #106.

Validation

• pnpm --filter dashboard exec vitest run src/components/repo/repo-markdown-files.test.ts
• pnpm --filter dashboard check-types
• pnpm --filter @diffkit/ui check-types
• targeted Biome checks for touched files
• git diff --check
• pnpm --filter dashboard build

[stylessh]

thanks @premsathisha for the contribution!

can you please share before/after images?

[premsathisha]

Here is the before and after from one of my repo's @stylessh:

[stylessh]

Thank you!

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	diffkit	f8a35a5	Apr 14 2026, 01:23 AM