THE KEY BAG: Every fashion show season has one. In September, it was the Cambridge Satchel in neon shades of pink, green, yellow and tangerine, and any color of Miu Miu’s glitter peep toe booties. But the fashion flock — at least those at the shows — has moved on. Although a token fluorescent messenger has been seen on the shoulders of a few outside Lincoln Center or Milk Studios, the “It” bag this season is the Celine luggage tote. The oversize bag, once coveted due to limited retailers that carry the French line, is clearly now more widely available. With an average price tag of $2,500, give or take a few hundred for varying sizes and fabrications, the bag follows in the footsteps of Proenza Schouler’s PS1, Givenchy’s Nightingale and YSL’s Muse.