Artifacts now flag when they're shared from another organization

What

• Artifact reads now track whether an artifact is external, meaning it was shared from a different organization than your own, and show distinct messaging for that case ("shared with you from another organization" / noting the author is outside your organization), separate from the existing public/owner/writer distinctions.
• When artifact data comes back marked with mode external, an explicit external: true flag is now passed along through both the artifact-read and comments-list code paths, rather than being dropped.
• A new typePage parameter (defaulting to "none") controls whether the artifact's "type contract" text (a description of the artifact's structure) is fetched and included when reading it, so that text isn't refetched on every single read.

Why

This makes it clearer to users and to Claude when an artifact originated outside their own organization, and avoids unnecessary repeated fetching of type-contract text on every artifact read.