It is quite a stretch to say that the Tories “have a deal lined up to make a smooth exit from the EU”.
They have a withdrawal agreement which means that nothing changes in practice until the end of 2020 - I guess that is what the markets are reacting to. Compared with “we’re leaving in March, no - July, no October, no January” I guess 12months where they can get on with life is a welcome change.
But we have nothing concrete in place for the end of December next year - and despite the optimism of senior Tories that they will conclude a trade deal in that time, pretty much no-one on the EU side, nor any experienced trade negotiator seems to agree with them.
So we will either drop completely out of the EU without a deal at the end of 2020 or Johnson will be forced into more embarrassing U-turns, neither of which seem much use from the point of “stability”.