root-relative markdown links resolve to npmx host

[BittuBarnwal7479]

Describe the bug

On package readme.ts page, root-relative markdown links such as /CONTRIBUTING.md are resolved as local npmx routes instead of repository files.

For example, on:
https://npmx.dev/package/astro page

Clicking a readme.ts link to /CONTRIBUTING.md opens:
https://npmx.dev/CONTRIBUTING.md

Instead, it should open the package repository’s rendered markdown file, for example:
https://github.com/withastro/astro/blob/HEAD/CONTRIBUTING.md

Additional context

Steps to Reproduce

1. Open https://npmx.dev/package/astro
2. Find a README link that points to /CONTRIBUTING.md
3. Click the link

Recording.2026-06-17.174239.mp4

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