4. Define a resource

A resource is read-only DATA the app can pull in for context, and it's identified by a URI (the scheme says where it lives — `file://`, `postgres://`, etc.). The client fetches it with `resources/read`. Give it a `uri` (required), a human `name`, and a `mimeType` so the client knows how to interpret the bytes. Unlike a tool, a resource takes no action — it just supplies content.