Quick skill activation via '+' button and conversation UI

Users should be able to upload and toggle skills directly from the conversation interface. Specifically:

The + button should allow uploading/activating skills
A "Use skills" dropdown (similar to "Use style") with toggles for each skill
Selecting a skill would add a system message notifying Claude the user intends to use it and include the skill in context

This mirrors the existing Styles UX and would make skills much more accessible.