fix: restore default export for Docusaurus plugin compatibility

Docusaurus requires plugins to have a default export. This was removed
in a previous commit but caused plugin loading to fail with:
"normalizedPluginConfig.plugin is not a function"

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

Author: scalvert

src/index.ts

@@ -1,3 +1,6 @@
+// Default export for Docusaurus plugin loading (requires default export)
+export { default } from './plugin/docusaurus-plugin.js';
+// Named export for explicit imports
 export { default as mcpServerPlugin } from './plugin/docusaurus-plugin.js';
 export { McpDocsServer } from './mcp/server.js';