docs: add project description to CHANGELOG.md

[Copilot]

CHANGELOG.md had no preamble — it jumped straight into version entries with no context about what the project is or what conventions the file follows.

Changes

• Added a brief project description after the # Changelog heading identifying @stackone/ai and its supported AI frameworks (OpenAI, Anthropic, Vercel AI SDK, MCP)
• Added standard references to Keep a Changelog and Semantic Versioning

Original prompt

Create a simple CHANGELOG.md file with a brief description of the project. The file should document the project changes, versions, and updates in a clear and organized manner following standard changelog conventions. Include an initial version entry with basic information about the stackone-ai-node project.

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Summary by cubic

Add a preamble to CHANGELOG.md that describes the @stackone/ai Node.js SDK (unified interface for SaaS actions) and lists supported frameworks (OpenAI, Anthropic, Vercel AI SDK, MCP). Also clarifies the changelog follows Keep a Changelog and Semantic Versioning.

^{Written for commit 00c203b. Summary will update on new commits.}