* feat(settings): group agents and skills by subfolder in sidebar
- Server: fix getUserAgentPath() to walk subfolders so grouped agent
layouts (e.g. agents/business/ceo.md) are correctly resolved
- Store: add 'group' field to AgentWithExtras and DiscoveredSkill,
parsed from file path at load time
- UI: add collapsible SidebarGroup component with localStorage-persisted
expand/collapse state
- AgentsSidebar: render custom agents grouped by subfolder name
- SkillsSidebar: render project/user skills grouped by domain folder
- Ungrouped items (flat root) fall through and render normally
* fix(settings): normalize group paths and add agent lookup caching parity
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Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>
