fix(pr): GitCafe attachments render in pull request bodies#31741

Problem

GitCafe writes uploaded screenshots into PR bodies as root-relative links: ![…](/api/attachments/attach_…). In the PR panel, web classified the path as a file on the server's machine, and mobile rejected it for not being http(s). Both showed "Image unavailable" for every GitCafe attachment, for example on #31251.

Change

• New resolveChangeRequestMediaUrl in client-runtime: a root-relative URL in a change request body means the forge's origin, never a local path. It ignores protocol-relative URLs (//, /\) and leaves workspace-relative paths alone.
• Web: a remark plugin, added to the per-repository PR markdown plugin list, rewrites image and definition URLs against the repository URL. Markdown offsets are unchanged, so checkbox write-back still works.
• Mobile: PullRequestMarkdownRepository passes the change request URL down. Image render, press-to-preview and the gallery resolve hrefs the same way.

Attachments on private GitCafe repos would still need an authenticated fetchMedia in the GitCafe driver, as GitHub has. Not done here; public attachments load directly.

Verification

• vp run check:changed: all pass. New regression rows in pullRequestMarkdown.test.ts.
• Web dev server, PR panel on #31251:

Mobile was not run on a device. It shares the resolver and only changes which href it passes.

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