Let $C$ be the curve parameterized by $\mathbf{r}(t) = \langle \cos t, \sin t \rangle$ for $0 \le t \le \pi$. For the vector field $\mathbf{F}(x, y) = \langle -y, x \rangle$, what is the integrand $\mathbf{F}(\mathbf{r}(t)) \cdot \mathbf{r}'(t)$ when setting up the line integral $\int_C \mathbf{F} \cdot d\mathbf{r}$?