feat: Add plugin-dev toolkit for comprehensive plugin development

Adds the plugin-dev plugin to public marketplace. A comprehensive toolkit for
developing Claude Code plugins with 7 expert skills, 3 AI-assisted agents, and
extensive documentation covering the complete plugin development lifecycle.

Key features:
- 7 skills: hook-development, mcp-integration, plugin-structure, plugin-settings,
  command-development, agent-development, skill-development
- 3 agents: agent-creator (AI-assisted generation), plugin-validator (structure
  validation), skill-reviewer (quality review)
- 1 command: /plugin-dev:create-plugin (guided 8-phase workflow)
- 10 utility scripts for validation and testing
- 21 reference docs with deep-dive guidance (~11k words)
- 9 working examples demonstrating best practices

Changes for public release:
- Replaced all references to internal repositories with "Claude Code"
- Updated MCP examples: internal.company.com → api.example.com
- Updated token variables: ${INTERNAL_TOKEN} → ${API_TOKEN}
- Reframed agent-creation-system-prompt as "proven in production"
- Preserved all ${CLAUDE_PLUGIN_ROOT} references (186 total)
- Preserved valuable test blocks in core modules

Validation:
- All 3 agents validated successfully with validate-agent.sh
- All JSON files validated with jq
- Zero internal references remaining
- 59 files migrated, 21,971 lines added

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

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

.claude-plugin/marketplace.json

@@ -123,6 +123,17 @@
       },
       "source": "./plugins/hookify",
       "category": "productivity"
+    },
+    {
+      "name": "plugin-dev",
+      "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.",
+      "version": "0.1.0",
+      "author": {
+        "name": "Daisy Hollman",
+        "email": "daisy@anthropic.com"
+      },
+      "source": "./plugins/plugin-dev",
+      "category": "development"
     }
   ]
 }