feat(claude-agent-sdk): add Claude Agent SDK as a provider

docs/integrations/claude_agent_sdk.md

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+# Claude Agent SDK
+
+This guide shows you how to use instructor with the Claude Agent SDK for structured outputs with agentic capabilities.
+
+## Installation
+
+```bash
+pip install instructor claude-agent-sdk
+```
+
+Make sure you have:
+- `ANTHROPIC_API_KEY` environment variable set
+- Claude Code CLI installed
+- Claude Code Max subscription with API access
+
+## Quick Start
+
+```python
+from instructor import from_claude_agent_sdk
+from pydantic import BaseModel
+import anyio
+
+class User(BaseModel):
+    name: str
+    age: int
+
+async def main():
+    client = from_claude_agent_sdk()
+
+    user = await client.create(
+        response_model=User,
+        messages=[{"role": "user", "content": "Extract: John is 25 years old"}]
+    )
+    print(user.name)  # John
+    print(user.age)   # 25
+
+anyio.run(main)
+```
+
+## Features
+
+### Guaranteed JSON Schema Compliance
+
+The Claude Agent SDK provides built-in JSON schema validation through its `output_format` option. This integration automatically:
+
+1. Converts your Pydantic model to a JSON schema
+2. Passes the schema to the Claude Agent SDK
+3. Validates the response with Pydantic
+4. Returns a type-safe model instance
+
+### Automatic Message Conversion
+
+The integration accepts instructor's standard message format and automatically converts it to a prompt for the Claude Agent SDK:
+
+```python
+# Standard instructor message format
+user = await client.create(
+    response_model=User,
+    messages=[
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "Extract user info from: John, age 25"}
+    ]
+)
+```
+
+### Retry Support
+
+The integration includes retry support for validation errors:
+
+```python
+user = await client.create(
+    response_model=User,
+    messages=[{"role": "user", "content": "Extract user info..."}],
+    max_retries=3  # Retry up to 3 times on validation errors
+)
+```
+
+## Examples
+
+### Contact Extraction
+
+```python
+from instructor import from_claude_agent_sdk
+from pydantic import BaseModel, Field
+from typing import Optional
+import anyio
+
+class ContactInfo(BaseModel):
+    name: str = Field(description="Full name of the person")
+    email: str = Field(description="Email address")
+    phone: Optional[str] = Field(default=None, description="Phone number")
+    company: Optional[str] = Field(default=None, description="Company name")
+
+async def main():
+    client = from_claude_agent_sdk()
+
+    email_text = """
+    Hi, I'm Sarah Johnson from TechCorp Inc.
+    Reach me at sarah.johnson@techcorp.com or (555) 123-4567.
+    """
+
+    contact = await client.create(
+        response_model=ContactInfo,
+        messages=[
+            {"role": "user", "content": f"Extract contact info:\n\n{email_text}"}
+        ]
+    )
+
+    print(f"Name: {contact.name}")
+    print(f"Email: {contact.email}")
+    print(f"Phone: {contact.phone}")
+    print(f"Company: {contact.company}")
+
+anyio.run(main)
+```
+
+### Sentiment Analysis
+
+```python
+from instructor import from_claude_agent_sdk
+from pydantic import BaseModel, Field
+from typing import List
+import anyio
+
+class SentimentAnalysis(BaseModel):
+    sentiment: str = Field(description="positive, negative, or neutral")
+    confidence: float = Field(ge=0.0, le=1.0)
+    key_phrases: List[str] = Field(description="Key phrases influencing sentiment")
+
+async def main():
+    client = from_claude_agent_sdk()
+
+    review = "This product is amazing! Best purchase I've ever made."
+
+    result = await client.create(
+        response_model=SentimentAnalysis,
+        messages=[
+            {"role": "user", "content": f"Analyze sentiment:\n\n{review}"}
+        ]
+    )
+
+    print(f"Sentiment: {result.sentiment}")
+    print(f"Confidence: {result.confidence:.2%}")
+    print(f"Key Phrases: {', '.join(result.key_phrases)}")
+
+anyio.run(main)
+```
+
+### Entity Extraction
+
+```python
+from instructor import from_claude_agent_sdk
+from pydantic import BaseModel, Field
+from typing import List
+import anyio
+
+class Entity(BaseModel):
+    name: str
+    type: str  # person, organization, location, date, etc.
+
+class EntityExtraction(BaseModel):
+    entities: List[Entity]
+    summary: str
+
+async def main():
+    client = from_claude_agent_sdk()
+
+    text = """
+    Apple CEO Tim Cook announced a meeting with French President Macron
+    in Paris on March 15, 2025.
+    """
+
+    result = await client.create(
+        response_model=EntityExtraction,
+        messages=[
+            {"role": "user", "content": f"Extract entities:\n\n{text}"}
+        ]
+    )
+
+    for entity in result.entities:
+        print(f"- {entity.name} ({entity.type})")
+
+anyio.run(main)
+```
+
+## Key Differences from Other Providers
+
+1. **Async Only**: The Claude Agent SDK integration is async-only since the underlying SDK uses async iterators.
+
+2. **Agentic Capabilities**: The Claude Agent SDK can perform agentic tasks with tool use, making it suitable for complex multi-step workflows.
+
+3. **Built-in Schema Validation**: The SDK validates JSON output against the schema before returning, providing an extra layer of reliability.
+
+## Configuration Options
+
+You can pass additional options when creating the client:
+
+```python
+client = from_claude_agent_sdk(
+    model="claude-sonnet-4-20250514",  # Optional: specify model
+    max_tokens=1024,                    # Optional: token limit
+    temperature=0.0,                    # Optional: temperature
+)
+```
+
+## Limitations
+
+- Async-only (use `anyio.run()` or `asyncio.run()` to run async code)
+- Requires Claude Code CLI installation
+- Requires Claude Code Max subscription