Add Claude Code plugin with "Create MCP App" skill (#256)

Add a Claude Code plugin (`plugins/mcp-apps/`) for distribution via the
plugin marketplace. The plugin provides the "Create MCP App" skill that
guides the agent through building MCP Apps with interactive UIs.

The skill includes:
- Instructions to clone SDK repo for reference templates
- Framework selection guidance (React, Vanilla JS, Vue, Svelte, etc.)
- Tool visibility configuration (`_meta.ui.visibility`)
- Host styling integration patterns
- Testing workflow using `basic-host`
- Common mistakes to avoid

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

plugins/mcp-apps/.claude-plugin/plugin.json

@@ -0,0 +1,11 @@
+{
+  "name": "mcp-apps",
+  "version": "0.1.0",
+  "description": "Claude Code skill for building MCP Apps with interactive UIs",
+  "author": {
+    "name": "MCP Apps"
+  },
+  "repository": "https://github.com/anthropics/mcp-ext-apps",
+  "license": "MIT",
+  "keywords": ["mcp", "mcp-apps", "model-context-protocol", "ui", "interactive"]
+}