Write migration and release documentation

[jozefizso]

Item 10: Write migration and release documentation

Suggested repository: appdmg/appdmg-cli

Created issue: #9

Project item: added to appdmg

Goal

Document the move from unscoped legacy packages to the @appdmg/* dependency
tree and the new Node.js 24 support policy.

Requirements

• Explain new package names.
• Explain executable rename to appdmg-cli.
• Explain no appdmg binary alias.
• Explain Node.js >=24.
• Document the preserved library EventEmitter API.
• Document schema export and JSON Schema 2020-12.
• Document minimum validation behavior and future schema-validation plan.
• Document stable error codes.
• Document platform support and non-Darwin runtime failure behavior.

Acceptance Criteria

• README examples use @appdmg/cli, appdmg-cli, and @appdmg/appdmg.
• Migration notes list all intentional breaking changes.
• Release notes are ready for first scoped 1.0.0 packages.