A 68-year-old woman is evaluated in the outpatient neurology clinic for cognitive concerns. Her daughter reports that over the past 18 months, the patient has gradually become forgetful, has difficulty managing finances, and occasionally gets lost in familiar places. She is alert and oriented during the consultation, maintains good attention throughout the 30-minute visit, and does not hallucinate. Vital signs are normal. Mini-Cog score is 2/3. What is the most likely diagnosis, and what is the key clinical feature that excludes delirium?