Implements the unified catalog from plans/mcp-resources-prompts-design.md §4.2 (T2): CatalogItem gains kind/uri/mime_type/size keyed as {kind}:{id}; catalog_items() lists per kind gated by advertised capabilities with warn-and-degrade; mcp_search results carry kind; mcp_describe gains an optional kind param (default tool); write-only registry ServerCatalog removed.