docs(slides): clarify gradient background support

[hmonadjem]

Request

Document whether editable gradient backgrounds are supported by gws slides, and describe the supported alternatives when they are not.

When automating branded presentations, it would be useful to specify a gradient by color stops and direction instead of supplying a raster background image. The important distinction is whether the result remains an editable gradient in Google Slides.

What I checked

On 7 September 2026, the public Slides v1 discovery document exposes:

• PageBackgroundFill: propertyState, solidFill, stretchedPictureFill.
• ShapeBackgroundFill: propertyState, solidFill.
• No schema names containing gradient.

Sources:

• https://slides.googleapis.com/$discovery/rest?version=v1
• https://developers.google.com/workspace/slides/api/reference/rest/v1/presentations.pages#PageBackgroundFill
• https://developers.google.com/workspace/slides/api/reference/rest/v1/presentations.pages/shapes#ShapeBackgroundFill

This appears to be an upstream Slides API limitation, not a field that the CLI is failing to expose. I have not attempted to send an undocumented gradient field or established a server-side rejection.

Suggested documentation

Please add a short note to the Slides skill/help explaining:

1. Whether there is any supported API path for creating or editing native gradients that is absent from the schemas above.
2. If not, that a CLI flag alone cannot provide native gradient support without an upstream API change.
3. The supported alternatives and their limitations, such as a solid fill or a PNG background, which does not remain an editable gradient.
4. Where users should track or request native gradient support upstream.

An optional helper that renders configured gradient stops into an image could be useful, but it should explicitly describe the output as a raster background rather than an editable gradient.