fix: add plugin.json manifest for Claude Code skill loading (#126)

Add the required plugin.json file to .claude-plugin/ directory to enable
Claude Code to properly load the UI/UX Pro Max skill.

The marketplace.json file is only used for marketplace indexing, while
plugin.json is required for the runtime plugin loading mechanism.

Includes full metadata (version, description, author, keywords) aligned
with the existing marketplace.json for consistency.

Fixes #123

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -0,0 +1,11 @@
+{
+  "name": "ui-ux-pro-max",
+  "version": "2.0.1",
+  "description": "Professional UI/UX design intelligence for AI coding assistants. Includes searchable databases of styles, colors, typography, charts, and UX guidelines for React, Next.js, Vue, Svelte, SwiftUI, React Native, Flutter, Tailwind, and shadcn/ui.",
+  "author": {
+    "name": "nextlevelbuilder"
+  },
+  "license": "MIT",
+  "keywords": ["ui", "ux", "design", "styles", "typography", "color-palette", "accessibility", "charts", "components"],
+  "skills": ["./.claude/skills/ui-ux-pro-max"]
+}