docs: add CHANGELOG.md

[montfort]

Summary

• Add CHANGELOG.md following Keep a Changelog 1.1.0
• Covers all releases: v0.1.0, v0.1.1, v0.2.0
• Includes comparison links between versions

Test plan

• Verify format matches keepachangelog spec
• Verify comparison links resolve correctly on GitHub

🤖 Generated with Claude Code

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