destroyable rope?

If I have a crane magnet hanging from a crane, is it possible (if its
attached via a rope) to make the rope breakable? So I can shoot
the rope and drop the magnet on someone?

Yes, it is possible.

Make two small brushes, with the nodraw texture on all sides, and place
one at either end of the rope. For this example, I'm going to
name the top one brush1, and the bottom one brush2. Make both
func_breakable entities. Under brush1's Output tab, put click
Add, and put "on break", target the rope entity that brush1 is
overlapping (eg, move_rope1), "break". This causes move_rope1 to
be severed when this invisible brush is broken. Under each brush,
set the value required to break to a low value, so that it'll break
under any bullet.

Do the same with brush2, except under it's output, target keyframe_rope1, or whatever that entity is called.

You may also have to check the "breakable" section under the move_rope
and keyframe_rope's properties, although I don't believe so.