fix(plugin-dev): add missing .claude-plugin/plugin.json

The plugin-dev plugin was missing its required plugin.json manifest file,
causing the plugin to fail loading. This adds the missing configuration
file following the same format as other official plugins.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/plugin-dev/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "plugin-dev",
+  "description": "Plugin development toolkit with skills for creating agents, commands, hooks, MCP integrations, and comprehensive plugin structure guidance",
+  "author": {
+    "name": "Anthropic",
+    "email": "support@anthropic.com"
+  }
+}