/design reworked into routed modes, plus a new /slides command

What

• The /design slash command's dispatch logic was rewritten to resolve a mode via a new router into one of consent, types, canvas, or hub, each with its own description, menu text, argument hint, and file-loading behavior (canvas mode loads design canvas files). The command also gained subcommandsBareOnly and survivesBundledKillSwitch flags, and its policy gate is now skipped for the types and canvas modes.
• A new /slides command is added alongside /design, letting you create a Slides deck artifact from a brief the same way /design creates a Design artifact. Both commands instruct the model to look up the matching published Artifact type and create a new artifact from it.

Why

Routing /design into distinct modes lets it handle browsing types, working with canvas files, and consent separately instead of one undifferentiated command, and /slides extends the same brief-to-artifact workflow to slide decks.