Draft slide template description

Purpose

Every template folder in the workspace's template library ships a description.md that tells a future deck builder what the slide looks like and when to reach for it. Freshly ingested templates carry only a TODO stub. This skill replaces that stub with a short, accurate description.

The point is selection speed: when someone builds a deck, they skim these files to pick the right slide. So the description must be honest about the real layout and useful for "should I use this one or not", not a generic blurb.

The screenshot is the source of truth. Describe what is really on the slide, not what the name suggests. If there is no screenshot (LibreOffice was not installed at ingest time), fall back to the fields and geometry, and say the description was drafted from fields rather than a render.