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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "antigravity-awesome-skills",
-  "version": "4.5.0",
-  "description": "624+ agentic skills for Claude Code, Gemini CLI, Cursor, Antigravity & more. Installer CLI.",
+  "version": "4.6.0",
+  "description": "626+ agentic skills for Claude Code, Gemini CLI, Cursor, Antigravity & more. Installer CLI.",
   "license": "MIT",
   "scripts": {
     "validate": "python3 scripts/validate_skills.py",