Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false; the description's 'Read-only' prefix largely restates them. It does add the useful scoping fact that results can be constrained to a folder, but discloses nothing about volume, ordering, or permission needs. With annotation coverage present, a 3 fits.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.