New file-based artifact types: slides and design

What

• A new slides artifact type stores each slide as its own HTML file (project/slides/<id>.html), with a project/deck.json index describing the deck's order, sections and fonts.
• A new design artifact type stores each artboard as its own file under project/, with a project/canvas.json index.
• The instructions Claude Code gives the model for these file-based types were rewritten to explain that content goes into files under project/, tracked with a createdOnFiles marker object, and published together in one step.

Why Previously, artifact types like these worked through an older "store document" model. Storing slides and designs as individual files with an index lets Claude Code build and update multi-file artifacts (a slide deck, a set of design artboards) more directly, and lets the model describe changes file-by-file instead of as one document blob.