Artifacts tool: icon replaces favicon, and read/list report editing access

What

• The Artifacts tool's favicon parameter (an emoji, e.g. "one or two emoji") is deprecated in favor of a new icon parameter that takes a single descriptive word like chart, calendar, recipe, code, or map instead of an emoji, and must not be a product or brand name.
• Publishing an artifact no longer requires a favicon at all; only file_path is required now.
• The publish result now reports whether the icon was accepted, dropped by the server, fell back to the old favicon emoji, or was never sent, via new response fields (iconDropped, icon, faviconSent).
• Reading or listing artifacts now reports whether you have edit ("writer") access to a shared artifact. Only artifacts you can write to can be updated directly; otherwise Claude has to publish a separate copy. Artifacts shared from another organization may be missing from the listing entirely.

Why

This tells Claude up front whether it can update an existing shared artifact or must create a new one, and simplifies icon selection to a plain word instead of picking an emoji.