The US economy, hobbled by last fall’s 43-day government shutdown , advanced at an unexpectedly sluggish 0.7% annual rate from October through December, the Commerce Department reported Friday in a big downgrade of its initial estimate. Growth in gross domestic product — the nation’s output of goods and services — was down sharply from 4.4% in last year’s third quarter and 3.8% in the second. And the fourth-quarter number was half the government’s  first estimate of 1.4% ; economists had