Add v0.1.0 changelog entry for Python SDK with comprehensive release documentation

Document breaking changes including ClaudeCodeOptions rename, system prompt changes, and settings isolation. Include new features like programmatic subagents, session forking, and granular settings control.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ashwin-ant

CHANGELOG.md

@@ -1,5 +1,52 @@
 # Changelog
 
+All notable changes to the Claude Agent SDK will be documented in this file.
+
+## 0.1.0 - 2025-09-29
+
+Introducing the Claude Agent SDK! The Claude Code SDK has been renamed to better reflect its capabilities for building AI agents across all domains, not just coding.
+
+### Breaking Changes
+
+#### Type Name Changes
+- **ClaudeCodeOptions renamed to ClaudeAgentOptions**: The options type has been renamed to match the new SDK branding. Update all imports and type references:
+  ```python
+  # Before
+  from claude_agent_sdk import query, ClaudeCodeOptions
+  options = ClaudeCodeOptions(...)
+
+  # After
+  from claude_agent_sdk import query, ClaudeAgentOptions
+  options = ClaudeAgentOptions(...)
+  ```
+
+#### System Prompt Changes
+- **Merged prompt options**: The `custom_system_prompt` and `append_system_prompt` fields have been merged into a single `system_prompt` field for simpler configuration
+- **No default system prompt**: The Claude Code system prompt is no longer included by default, giving you full control over agent behavior. To use the Claude Code system prompt, explicitly set:
+  ```python
+  system_prompt={"type": "preset", "preset": "claude_code"}
+  ```
+
+#### Settings Isolation
+- **No filesystem settings by default**: Settings files (`settings.json`, `CLAUDE.md`), slash commands, and subagents are no longer loaded automatically. This ensures SDK applications have predictable behavior independent of local filesystem configurations
+- **Explicit settings control**: Use the new `setting_sources` field to specify which settings locations to load: `["user", "project", "local"]`
+
+For full migration instructions, see our [migration guide](https://docs.claude.com/en/docs/claude-code/sdk/migration-guide).
+
+### New Features
+
+- **Programmatic subagents**: Subagents can now be defined inline in code using the `agents` option, enabling dynamic agent creation without filesystem dependencies. [Learn more](https://docs.claude.com/en/api/agent-sdk/subagents)
+- **Session forking**: Resume sessions with the new `fork_session` option to branch conversations and explore different approaches from the same starting point. [Learn more](https://docs.claude.com/en/api/agent-sdk/sessions)
+- **Granular settings control**: The `setting_sources` option gives you fine-grained control over which filesystem settings to load, improving isolation for CI/CD, testing, and production deployments
+
+### Documentation
+
+- Comprehensive documentation now available in the [API Guide](https://docs.claude.com/en/api/agent-sdk/overview)
+- New guides for [Custom Tools](https://docs.claude.com/en/api/agent-sdk/custom-tools), [Permissions](https://docs.claude.com/en/api/agent-sdk/permissions), [Session Management](https://docs.claude.com/en/api/agent-sdk/sessions), and more
+- Complete [Python API reference](https://docs.claude.com/en/api/agent-sdk/python)
+
+---
+
 ## 0.0.22
 
 - Introduce custom tools, implemented as in-process MCP servers.