Yes this is bad for your engine for a multitude of reasons. In fact, it's not good to run your engine at full throttle for extended periods of time unless the engine is designed to take it, whether it's cold or warmed up. Believe me when I say your little 1.2l Agila engine is not designed to take that kind of stress.
I'll start by suggesting that engines, while having a red line are not meant to live at the red line. In other words, the red line is a point not to go past, but also not a place to keep your engine running at. Very few engines are designed that way (NASCAR and F1 engines are exceptions, that's for sure ... but they are completely warmed up before they are run at the engine speeds they are required to run at).
Another thing to think about is metal can stand only so much stress before it will fail. The stress on metal is accumulative. I've posted this graph before, but I'll post it again because it bears repeating:
This graph is pulled from a book written by Carroll Smith called Engineer to Win. The graph shows that metal has a fatigue limit, or a point at which repeated cycles will have little to no effect on a metal part (meaning a part will last a long, long time if the stress put upon it is below this level). Above this level, the stress put upon the part detracts from its longevity ... the higher the stress, the sooner the part failure. Like I said, this effect is accumulative ... it adds up over time. About this, Carroll Smith said:
Simplistically put, under repeated (cyclic as opposed to continuous) stress the capacity of a metal to withstand stress gradually diminishes and, in most cases, cannot be restored. Metals which are subjected to fluctuating loads can and do break after a finite number of load cycles (of, more accurately, stress cycles) in which the loads applied and the resultant stresses imposed are always below the ultimate strength of the metal. This type of failure is termed "fatigue failure."
Basically what I was saying, only he says it much better.
Until an engine is fully warmed up, the oil doesn't flow as well as it should through the engine. By putting the extra stress on the engine, you are not giving the engine the benefit of the free flowing oil it deserves. This causes additional stress as well as extra wear on the bearings, crankshaft journals, cam journals and lobes, plus the rest of the valve train. Seals don't seal as well when cold, so wear out faster due to more drag on their surfaces.
There is no magic number I can tell you to keep it under as far as engine speed goes. Just remember what I've said above ... The higher the stress, the shorter the engine will live. The important thing here is to ensure you are not abusing your engine (as it sounds like you are). Pull out easy and accelerate easy as well. If you have to floor it to get up to speed, you may need to think about varying your route to allow for the warm up period. If you have to keep it floored to keep up with traffic, either consider getting a vehicle which can handle the speed of traffic or take a side road which will allow you to travel at a slower speed.