<image>First simplify, then evaluate:

$$(1)$$ First simplify, then evaluate: $$a(a-4)-(a+6)(a-2)$$, where $$a=- \dfrac{1}{2} $$ .

$$(2)$$ $$\left(1- \dfrac{1}{a-1}\right) ÷ \dfrac{a^{2}-4a+4}{a^{2}-a} $$, where $$a=-1$$.