docs: fix plugin installation commands in README

Use correct CLI syntax: claude plugins marketplace add, claude plugins install plugin@marketplace

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: canyugs

README.md

@@ -43,10 +43,11 @@ curl -X POST "https://your-rag-service/api/learn" \
 
 ## Agent Integration (Claude Code Plugin)
 
-Install the plugin to give your Claude Code agent search, learn, and report skills:
+Add the marketplace and install the plugin:
 
 ```bash
-claude plugin add zeabur/rag-service
+claude plugins marketplace add zeabur/rag-service
+claude plugins install rag-service@rag-service
 ```
 
 Then configure the connection: