docs: add project description and Keep a Changelog preamble to CHANGELOG.md

[Copilot]

CHANGELOG.md was auto-generated by release-please with no project context or format declaration at the top.

Changes

• Project description: Added a one-line description of @stackone/ai as a Node.js SDK for the StackOne AI platform
• Keep a Changelog preamble: Added standard "All notable changes..." boilerplate with links to Keep a Changelog and Semantic Versioning

Existing auto-generated version history is preserved unchanged.

Original prompt

Create a CHANGELOG.md file for the stackone-ai-node project. The file should include:

1. A brief description of the project at the top
2. A section for the initial version with basic changelog entries
3. Follow standard CHANGELOG format (Keep a Changelog style)
4. Include sections for different types of changes (Added, Changed, Fixed, etc.)
5. A brief description explaining what stackone-ai-node is: a Node.js SDK for the StackOne AI platform that provides integration capabilities

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Summary by cubic

Add a brief description of @stackone/ai and a standard Keep a Changelog + Semantic Versioning preamble to CHANGELOG.md. This clarifies what the SDK is and the format/versioning we follow.

^{Written for commit c1bb806. Summary will update on new commits.}