Add two new agent tools for deep element inspection and property mutation:
- get_element: returns full type-specific metadata for any timeline element
(video, image, text, graphic, sticker, audio, effect)
- update_clip: mutates any visual property on an element including masks,
opacity, transform, blendMode, hidden, volume, muted, and name
Masks are handled as a nested property with add/update/remove actions,
aligned with the data model where masks are properties of elements rather
than standalone timeline entities.
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