Automations: Set A Fulfillment's Details When It's Created

Automations can now set fulfillment details automatically when a new fulfillment lands. Pair the new "Set Fulfillment Dynamic Field" action with the "fulfillment created" trigger, pick a field, and set the value once — every fulfillment created under that rule picks it up.

Four fields are supported: instructions, gift message, third-party billing (on/off), and carrier service. Carrier service routes through the shipping-instructions update path so the requested carrier name and id land together; the other three go through the fulfillment-details update path.