Artifact Tool: Smarter url Parameter Guidance

The description for the url parameter in the Artifact tool has been rewritten to cover more update scenarios:

Before: "Pass when the user gives you a URL for an artifact not published in this session."

After: "Pass whenever the user wants to update an artifact this conversation did not publish — 'update my artifact', 'keep the same link', a pasted artifact URL — and find the URL with action: 'list' if you don't have it; without this, a conversation that didn't publish the artifact always mints a new URL."

This teaches Claude to proactively use action: "list" to locate the URL rather than waiting for the user to paste it.