I restarted with 110 kittens (yay for 40 paragon!) and am really missing my trading. I can't seem to find anywhere on the wiki or even in the code how to make my trade tab appear.
What trigger makes the trade tab show up on my tab bar?
The trading tab is unlocked the moment it is useful; specifically, it unlocks once the first race sends an emissary to your village. This leads to the obvious problem that you need the Explore button on that tab to unlock more. Therefore, the game helps you out a bit; on reaching Year 20, a single race is unlocked for trade, for free, which then unlocks the trading tab to discover more.