Remove skills.sh install option from README

Only the Plugin Marketplace method remains as the skill installation path.

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

Author: ya-luotao

README.md

@@ -128,11 +128,7 @@ gem install claude-agent-sdk
 
 ### Agentic Coding Skill
 
-If you're using [Claude Code](https://claude.ai/claude-code) or another agentic coding tool that supports [skills](https://skills.sh), you can install the SDK skill:
-
-**Option 1: Via Plugin Marketplace (recommended)**
-
-This repo is a Claude Code plugin marketplace. Add it once, then install the skill:
+If you're using [Claude Code](https://claude.ai/claude-code), this repo is a Claude Code plugin marketplace. Add it once, then install the skill:
 
 ```bash
 # Add the marketplace
@@ -142,12 +138,6 @@ This repo is a Claude Code plugin marketplace. Add it once, then install the ski
 /plugin install claude-agent-ruby@claude-agent-sdk-ruby
 ```
 
-**Option 2: Via skills.sh**
-
-```bash
-npx skills add https://github.com/ya-luotao/claude-agent-sdk-ruby --skill claude-agent-sdk-ruby
-```
-
 This skill teaches your AI coding assistant about the SDK's APIs, patterns, and best practices, making it easier to get help writing code that uses this SDK.
 
 ## Quick Start