Adding a plugin marketplace no longer silently overwrites an existing one

What

When you add a plugin marketplace from a url source, Claude Code now hardlinks the downloaded catalog into the cache instead of always renaming it, and:

• If a marketplace with that name is already registered from a different source, the add is refused with an error instructing you to run /plugin marketplace remove first.
• If finalizing the cache fails for another reason, a more explicit error is shown, explaining that adding a marketplace will never silently replace an existing one with the same name, and how to recover.

Why

This prevents one marketplace from quietly clobbering another that happens to share a name, which could otherwise replace plugins you already trust without warning.