[FEATURE]: Group/organize skills in the CLI picker instead of one flat list

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When skills are installed from multiple packs/collections (e.g. via npx skills add ...), they all land in the skill picker / slash-command menu as one long flat list. Once you've got skills from more than one collection, the list becomes long enough that finding a specific skill means scrolling and reading through everything rather than jumping straight to it ,the picker doesn't scale well past a handful of skills.

A few possible directions on how to fix this, roughly in order of how much UI work they'd take:

1. Fuzzy search/filter in the picker, matching against SKILL.md content, not just the skill name: as you type in the picker, it filters against each skill's full SKILL.md (name, description frontmatter, and body), not only the visible title. Matching on content means you could type something and land on the right skill even if that word isn't in its name.
2. Grouped headers/sections by source folder: still one scrollable list, but skills are clustered under a header per source directory, similar to how some pickers group by category.
3. Collapsible folder tree: full file-explorer-style nesting, expand/collapse per source. Most UI work, most scalable for people with many collections installed.

These aren't mutually exclusive.Happy to take a stab at implementing this myself if a maintainer can weigh in.