Artifact tool schema gains read_page_data, capabilities/contract, and typed-Artifact catalog fields

The schema that defines the artifact tool's parameters was rewritten and now documents several new pieces:

• read_page_data, an action that validates a page's declared data against a named interaction schema
• a reserved preflight.js file convention at the artifact root
• capabilities and contract fields for declaring what an artifact can do at runtime
• type_query, type, type_url, and auto_open fields for browsing and creating artifacts from published "types"
• a label field for named publishes