docs(changelog): add initial release notes

Added CHANGELOG.md capturing the 0.1.0 release details as requested.

Author: OhadAssulin

CHANGELOG.md

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+# Changelog
+
+## [0.1.0] - 2025-11-08
+### 🎉 First Public Release
+
+This is the first public release of **Headless Coder SDK**, an open-source framework that unifies multiple headless AI-coder SDKs — **OpenAI Codex**, **Anthropic Claude Agent SDK**, and **Google Gemini CLI (headless)** — under one consistent developer interface.
+
+### 🚀 Highlights
+- Unified `createCoder()` and thread API across Codex, Claude, and Gemini.
+- Standardized streaming via `runStreamed()` and structured output via `outputSchema`.
+- Shared permission + sandbox model (`read-only`, `workspace-write`, `danger-full-access`).
+- Thread resume support for Codex and Claude adapters.
+- Cooperative cancellation using `RunOpts.signal` or `thread.interrupt()`.
+- Modular adapter registration pattern:
+  ```ts
+  registerAdapter(CODEX_CODER, createCodexAdapter);
+  ```
+- Initial permission and sandbox enforcement layer.
+- Example suite for multi-provider workflows and structured output validation.
+
+### 📦 Packages included
+- `@headless-coder-sdk/core`  
+- `@headless-coder-sdk/codex-adapter`  
+- `@headless-coder-sdk/claude-adapter`  
+- `@headless-coder-sdk/gemini-adapter`  
+- `@headless-coder-sdk/examples`
+
+### 🧩 Developer Docs
+- [README](https://github.com/OhadAssulin/headless-coder-sdk#readme)
+- [Create Your Own Adapter guide](https://github.com/OhadAssulin/headless-coder-sdk/blob/main/docs/create-your-own-adapter.md)
+
+### 🧠 Notes
+This release establishes the unified adapter interface, event model, and sandboxing foundation for future integrations and features — such as structured streaming, granular permissions, and new AI-coder backends.
+
+---
+
+_© 2025 Ohad Assulin — MIT License_