Get the logged-in user (requestor) for this MCP session.

Call this when you need to know who is authenticated — for example when the user asks "who am I?", when a workflow needs the current user's ID, or before calling tools that act on behalf of the requestor (intro requests, relationship actions, workspace membership checks, etc.).

The returned user_id is the Keycloak subject (sub claim). This is the same identifier injected as user_id on workspace-scoped tool calls via request context.

Returns: The requestor's user_id plus basic profile fields from the access token when available (email, name).