docs: add advanced skills for Vitest, Pinia, and Vue built-ins

Add comprehensive reference documentation for:
- Vitest: environments, projects/workspaces, type testing, vi utilities
- Pinia: HMR, Nuxt integration, SSR setup
- Vue: built-in components (Transition, Teleport, Suspense, KeepAlive) and advanced directives

.agents/skills/web-design-guidelines/SYNC.md

@@ -0,0 +1,5 @@
+# Sync Info
+
+- **Source:** `vendor/web-design-guidelines/skills/web-design-guidelines`
+- **Git SHA:** `5847a7c7e79bab3e400cf47800b83449d7aea2d4`
+- **Synced:** 2026-03-16