add help docs and ?

docs/nodes/Gradient.md

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+# Gradient
+
+Compute the spatial gradient using a Sobel operator. Outputs the gradient magnitude, x-component, y-component, or azimuthal direction.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| gradient | DATA_FIELD | Gradient output in the selected component |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| component | dropdown | magnitude | Output component: magnitude (√(gx²+gy²)), x (horizontal gradient), y (vertical gradient), or azimuth (angle in degrees) |
+
+## Limitations
+
+- Gradient components are in units of z_unit/xy_unit; azimuth is in degrees.
+- Sobel operator uses a 3×3 kernel; sub-pixel features smaller than one pixel cannot be resolved.