A 68-year-old man with a history of hypertension presents to the ophthalmology outpatient department with gradual, painless vision loss over 6 months. He denies eye pain, redness, or halos around lights. On examination, visual acuity is 6/9 in both eyes. Intraocular pressure (IOP) is 28 mmHg OD and 26 mmHg OS (normal <21 mmHg). Anterior chamber depth is normal on slit-lamp examination. Gonioscopy reveals Shaffer grade III angles bilaterally. Optic disc shows a vertical cup-to-disc ratio of 0.8 with inferior neuroretinal rim thinning. Perimetry demonstrates a superior arcuate scotoma in the left eye. What is the most likely diagnosis?