hook, barb, gaff, brackets
鈎 centers on a curved or bent tool for catching, holding, or hanging: a hook, barb, or gaff. The bracket sense extends this idea to a supporting or enclosing hook-like shape.
鈎 combines 金 (metal) with 勾, which likely contributes both sound and the idea of a hook or curved shape. The character is associated with metal hooks.
Metal 金 on the left, and a hook-like shape 勾 on the right. Picture a metal hook used for fishing or hanging things.
For コウ, imagine a coat hanging on a metal hook: coat -> コウ, and the hook holds it securely.