Artifact reads now include Share menu access details

What

• When reading an artifact as its owner, the response can now include a shareMenu object listing which read access modes are assignable, plus optional externalSharing and externalInvites flags, derived from permission data.
• For owner-role artifacts that aren't type-locked and have a share menu, Claude Code now appends a descriptive text summary to the artifact's audience description, phrased like "(from the page's Share menu the user's plan and organization allow: ...)", noting whether a public link is unavailable and whether external email invites are turned on or off.

Why

This surfaces sharing and access permissions directly when reading an artifact, so it's clear who can see or be invited to it without checking the Share menu separately.