Implement Prewitt Edge Detector

Implement the Prewitt edge detector, one of the earliest edge detection methods (1970).

The Prewitt operator uses two 3×3 kernels to compute horizontal and vertical gradients:

$G_x = \begin{pmatrix} -1 & 0 & 1 \\ -1 & 0 & 1 \\ -1 & 0 & 1 \end{pmatrix} * I, \quad G_y = \begin{pmatrix} -1 & -1 & -1 \\ 0 & 0 & 0 \\ 1 & 1 & 1 \end{pmatrix} * I$

The edge magnitude and direction are: $G = \sqrt{G_x^2 + G_y^2}, \quad \theta = \arctan\left(\frac{G_y}{G_x}\right)$

Return both the magnitude map and the edge direction map (in degrees, 0-360).