docs: prepare v0.2.0 changelog

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Summary

• roll the current changelog state into a dedicated v0.2.0 section
• keep an empty Unreleased section at the top for the next release cycle
• capture the user-visible parser, sudo-signal, and release-documentation changes already merged to main

Scope

• docs-only change in CHANGELOG.md
• no source code, tests, or release assets changed

Verification

• confirmed the branch diff only touches CHANGELOG.md
• local CMake/ctest were not rerun because cmake is not available in the current shell environment
• merge validation will rely on the repository's GitHub Actions checks