chore(release): 4.6.0 - SPDD & Radix UI Design System

- Bump version to 4.6.0, update CHANGELOG and README counts to 626
- Fix listSkillIds to only include dirs with SKILL.md (SPDD excluded from catalog)
- Regenerate catalog and index after PR 47/48 merge

skills_index.json

@@ -3977,6 +3977,15 @@
     "risk": "unknown",
     "source": "unknown"
   },
+  {
+    "id": "radix-ui-design-system",
+    "path": "skills/radix-ui-design-system",
+    "category": "uncategorized",
+    "name": "radix-ui-design-system",
+    "description": "Build accessible design systems with Radix UI primitives. Headless component customization, theming strategies, and compound component patterns for production-grade UI libraries.",
+    "risk": "safe",
+    "source": "self"
+  },
   {
     "id": "rag-engineer",
     "path": "skills/rag-engineer",