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

The plugin-dev plugin is listed in .claude-plugin/marketplace.json but was the
only plugin in the plugins/ directory without a .claude-plugin/plugin.json
manifest, which is required for installation from the marketplace.

Adds the manifest with name, version, description, and author matching the
marketplace.json entry, consistent with all other plugins in this repo.

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

@@ -0,0 +1,9 @@
+{
+  "name": "plugin-dev",
+  "version": "0.1.0",
+  "description": "Comprehensive toolkit for developing Claude Code plugins. Includes 7 expert skills covering hooks, MCP integration, commands, agents, and best practices. AI-assisted plugin creation and validation.",
+  "author": {
+    "name": "Daisy Hollman",
+    "email": "daisy@anthropic.com"
+  }
+}