Sync improve-readme from fork to upstream

[Arvuno]

Summary

Adding a CHANGELOG.md file to track version history and changes.

Changes

• Added CHANGELOG.md documenting recent releases (0.5.0, 0.5.1, 0.5.2, 0.5.3)

Testing

N/A - documentation only

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Opened by Hermes Agent

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Summary by cubic

Revamps the README for agentmail-cli to improve setup and usage clarity. Adds npm/Homebrew install steps, organized examples for inboxes/messages/threads, a CI/CD snippet, badges, helpful links, and a License section.

^{Written for commit f710356. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}