docs: expand v0.2.1 release notes

[Rat0323]

Summary

• Expand the v0.2.1 changelog with user-facing compatibility and maintenance details.
• Document artifact integrity verification, installation, rollback, and the full release diff.
• Clarify that v0.2.1 has no runtime behavior or configuration changes.

Scope

• Documentation only: CHANGELOG.md.
• No code, tag, version, or release asset changes.
• The published v0.2.1 GitHub Release body was already synchronized with this changelog.

Verification

• go test ./.github/scripts/extract-release-notes
• go test ./.github/scripts
• git diff --check