feat: add Claude Code plugin package (#541)

Add separate plugin package for Claude Code plugin directory submission.

Structure:
- .claude-plugin/plugin.json - plugin metadata
- .mcp.json - MCP server configuration
- README.md - documentation with use case examples

packages/claude-plugin/.claude-plugin/plugin.json

@@ -0,0 +1,11 @@
+{
+    "name": "next-ai-drawio",
+    "version": "1.0.0",
+    "description": "AI-powered Draw.io diagram generation with real-time browser preview. Create flowcharts, architecture diagrams, and more through natural language.",
+    "author": {
+        "name": "DayuanJiang"
+    },
+    "repository": "https://github.com/DayuanJiang/next-ai-draw-io",
+    "homepage": "https://next-ai-drawio.jiang.jp",
+    "license": "Apache-2.0"
+}

packages/claude-plugin/.mcp.json

@@ -0,0 +1,8 @@
+{
+    "mcpServers": {
+        "drawio": {
+            "command": "npx",
+            "args": ["@next-ai-drawio/mcp-server@latest"]
+        }
+    }
+}

packages/claude-plugin/README.md

@@ -0,0 +1,107 @@
+# Next AI Draw.io - Claude Code Plugin
+
+AI-powered Draw.io diagram generation with real-time browser preview for Claude Code.
+
+## Installation
+
+### From Plugin Directory (Coming Soon)
+
+Once approved, install via:
+```
+/plugin install next-ai-drawio
+```
+
+### Manual Installation
+
+```bash
+claude --plugin-dir /path/to/packages/claude-plugin
+```
+
+Or add the MCP server directly:
+```bash
+claude mcp add drawio -- npx @next-ai-drawio/mcp-server@latest
+```
+
+## Features
+
+- **Real-time Preview**: Diagrams appear and update in your browser as Claude creates them
+- **Version History**: Restore previous diagram versions with visual thumbnails
+- **Natural Language**: Describe diagrams in plain text - flowcharts, architecture diagrams, etc.
+- **Edit Support**: Modify existing diagrams with natural language instructions
+- **Export**: Save diagrams as `.drawio` files
+- **Self-contained**: Embedded server, no external dependencies required
+
+## Use Case Examples
+
+### 1. Create Architecture Diagrams
+
+```
+Generate an AWS architecture diagram with Lambda, API Gateway, DynamoDB,
+and S3 for a serverless REST API
+```
+
+### 2. Flowchart Generation
+
+```
+Create a flowchart showing the CI/CD pipeline: code commit -> build ->
+test -> staging deploy -> production deploy with approval gates
+```
+
+### 3. System Design Documentation
+
+```
+Design a microservices e-commerce system with user service, product catalog,
+shopping cart, order processing, and payment gateway
+```
+
+### 4. Cloud Architecture (AWS/GCP/Azure)
+
+```
+Generate a GCP architecture diagram with Cloud Run, Cloud SQL, and
+Cloud Storage for a web application
+```
+
+### 5. Sequence Diagrams
+
+```
+Create a sequence diagram showing OAuth 2.0 authorization code flow
+between user, client app, auth server, and resource server
+```
+
+## Available Tools
+
+| Tool | Description |
+|------|-------------|
+| `start_session` | Opens browser with real-time diagram preview |
+| `create_new_diagram` | Create a new diagram from XML |
+| `edit_diagram` | Edit diagram by ID-based operations |
+| `get_diagram` | Get the current diagram XML |
+| `export_diagram` | Save diagram to a `.drawio` file |
+
+## How It Works
+
+```
+Claude Code <--stdio--> MCP Server <--http--> Browser (draw.io)
+```
+
+1. Ask Claude to create a diagram
+2. Claude calls `start_session` to open a browser window
+3. Claude generates diagram XML and sends it to the browser
+4. You see the diagram update in real-time!
+
+## Configuration
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `PORT` | `6002` | Port for the embedded HTTP server |
+| `DRAWIO_BASE_URL` | `https://embed.diagrams.net` | Base URL for draw.io (for self-hosted deployments) |
+
+## Links
+
+- [Homepage](https://next-ai-drawio.jiang.jp)
+- [GitHub Repository](https://github.com/DayuanJiang/next-ai-draw-io)
+- [MCP Server Documentation](https://github.com/DayuanJiang/next-ai-draw-io/tree/main/packages/mcp-server)
+
+## License
+
+Apache-2.0