First of all we need to INSTANTLY create a vehicle type on the battlefield.
For this,two ways: Paradrop(Super fast inviso plane,super fast drop speed,but behaves bad) or Droppod SW.

Set Droppod anim to INVISO,and the weapon used Damage= to 0,as not to show the anim.Also Report= to DUMMY

Then we come to the Unit created.
As I have asked,and all of you have answered,it's just impossible to make a unit with "DeployToFire=yes" autofire.
But it can Retaliate to fire its weapon,meaning that 'If anything attacks me,I will deploy.'

Then there it is.

Set the armor of it to special_2,as it is used only on spawned missiles
Give all your construction yards .or any building without a weapon the weapon to attack it,ranged 255,warhead only affects special_2,projectile not AA,as you can't let your construction yard kill spawned missiles.This is like a general edit.
Give your Builder unit the weapon to fight back.Also ranged 255,but warhead can simply be SA,as it is never fired.Also if anything else attacks it,it deploys.

Give the builder DontSayUnitLost=yes.Said to be in the patch,but not in the instructions.According to test,effective.

Warhead of the construction yard's weapon should have an empty anim,which loops a few time and then Next= to an anim with damage to kill the builder if it rests on ore or anyplace that cannot place a building.(Attention:maybe you should edit Fire2 warhead to let it deals no damage to buildings.I used NP extention by LH_Mouse,which allows customized anim warhead)
Or simply use a gattling system.Not tested but might be useful.

The Builder's death explosion can be set to some empty image,Report= set to EVA sound"cannot deploy here"