Twas the night before Christmas in this offseason of doubt when news broke that the Mariners were adding a free-agent hitter that might provide some clout. Early Sunday evening as many families gathered for holiday meals, the organization reached an agreement with free-agent catcher/designated hitter Mitch Garver on a two-year, $24 million contract pending a physical. An MLB source confirmed the report from ESPN’s Jeff Passan. Garver, who turns 33 on Jan. 15, is coming off a solid season with the Rangers. In 87 games with Texas, he posted a .270/.370/.500 slash line with 11 doubles, 19 homers,…