fix: add missing skills field to plugin.json so Claude Code loads skills

Without "skills": "./skills/", the Claude Code plugin installer never
registers the skill files and /frontend-slides produces 0 skills on
/reload-plugins. Fixes #60 #63 #64.

plugins/frontend-slides/.claude-plugin/plugin.json

@@ -9,5 +9,6 @@
   "homepage": "https://github.com/zarazhangrui/frontend-slides",
   "repository": "https://github.com/zarazhangrui/frontend-slides",
   "license": "MIT",
-  "keywords": ["presentations", "slides", "html", "design", "powerpoint", "animations"]
+  "keywords": ["presentations", "slides", "html", "design", "powerpoint", "animations"],
+  "skills": "./skills/"
 }