The giant drugstore chain CVS Health Corp. is nearing an agreement to acquire Oak Street Health Inc., which would rapidly expand the company’s footprint of primary-care doctors. This deal would balloon its network of medical facilities specializing in senior health. The deal is massive at roughly $10.5 billion and could be announced as early as next week. The transaction would follow CVS’s $8 billion agreement to acquire home-care provider Signify Health Inc. Oak Street has 160 centers across 21 states, focused on the care of patients enrolled in Medicare.