feat(skills): unified management architecture with SSOT and React Query

- Introduce SSOT (Single Source of Truth) at ~/.cc-switch/skills/
- Add three-app toggle support (Claude/Codex/Gemini) for each skill
- Refactor frontend to use TanStack Query hooks instead of manual state
- Add UnifiedSkillsPanel for managing installed skills with app toggles
- Add useSkills.ts with declarative data fetching hooks
- Extend skills.ts API with unified install/uninstall/toggle methods
- Support importing unmanaged skills from app directories
- Add v2→v3 database migration for new skills table structure

src/components/skills/SkillCard.tsx

@@ -12,10 +12,12 @@ import { Button } from "@/components/ui/button";
 import { Badge } from "@/components/ui/badge";
 import { ExternalLink, Download, Trash2, Loader2 } from "lucide-react";
 import { settingsApi } from "@/lib/api";
-import type { Skill } from "@/lib/api/skills";
+import type { DiscoverableSkill } from "@/lib/api/skills";
+
+type SkillCardSkill = DiscoverableSkill & { installed: boolean };
 
 interface SkillCardProps {
-  skill: Skill;
+  skill: SkillCardSkill;
   onInstall: (directory: string) => Promise<void>;
   onUninstall: (directory: string) => Promise<void>;
 }