Add claude-api skill (#515)

Documentation skill for building applications with the Claude API
and Agent SDK. Covers Python, TypeScript, Java, Go, Ruby, C#, PHP,
and cURL with language-specific guides for:

- Messages API basics, streaming, and error handling
- Tool use (tool runner and manual agentic loop)
- Structured outputs and adaptive thinking
- Batches and Files APIs
- Agent SDK patterns (Python/TypeScript)
- Model catalog and selection guidance

.claude-plugin/marketplace.json

@@ -41,5 +41,15 @@
         "./skills/webapp-testing"
       ]
     }
+    ,
+    {
+      "name": "claude-api",
+      "description": "Claude API and SDK documentation skill for building LLM-powered applications",
+      "source": "./",
+      "strict": false,
+      "skills": [
+        "./skills/claude-api"
+      ]
+    }
   ]
 }