The value of \( \tan^{-1}\!\left( \frac{\cos\!\left( \frac{15\pi}{4} \right)}{\sin\!\left( \frac{\pi}{4} \right)} - 1 \right) \) is equal to:
\( -\frac{\pi}{4} \)
\( -\frac{\pi}{8} \)
\( -\frac{5\pi}{12} \)
\(- \frac{4\pi}{9} \)