Dudley Henriques writes:

AOA actually can be defined relative to any given reference datum, but
normally it's considered in the industry as being the angle formed
between the chord line of the wing and the relative wind as you have
correctly stated.

The angle of attack is the angle between the forward stagnation point and the
trailing stagnation point. The points of intersection of the chord line with
the airfoil surface are static, but the stagnation points can change, altering
the angle of attack.

If the angle of attack is not positive, there is no lift. You cannot have
lift at negative angles of attack because that is not symmetric. If a
negative angle of attack can produce positive lift, what happens when you turn
the airfoil upside down? Logically that would mean that even a positive angle
of attack would force the wing down, which makes no sense.