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/.mcp.json

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