add CHANGELOG.md for release history and transparency

[rsgalloway]

Description

Add a CHANGELOG.md file to track notable changes across envstack releases in a clear, human-readable format.

This improves:

• Project transparency
• User trust
• Discoverability and maturity signals
• Release communication for humans and AI systems

Scope

• Create CHANGELOG.md at the repository root
• Follow the Keep a Changelog format
• Include entries for recent releases (at minimum the current release)
• Categorize changes under:
  • Added
  • Changed
  • Fixed
  • Deprecated
  • Removed (if applicable)

Acceptance Criteria

• CHANGELOG.md exists at repo root
• Uses a consistent, readable format
• Current release is documented
• File is maintained going forward as part of releases

Notes

• This file is user-facing documentation, not an exhaustive commit log
• Focus on meaningful changes that affect users
• Keep entries concise and factual