Resolve an internal link (href from doc_id) to its target doc id, so the editor can navigate name/path links (e.g. topics/pricing.md) in-app.

The document's own workspace wins. Failing that we look in the caller's OTHER workspaces, so a link still opens after its target was moved across — the one thing a cross-workspace move leaves behind. The returned workspace_id tells the client which workspace to switch to.