docs(changelog): replace inherited upstream history with real releases

The docs changelog still showed the pre-fork CLI's 0.2.0-0.17.1 history.
Replace it with this repository's actual published releases (0.2.0-0.5.1,
all 2026-08-03), synced from apps/pythinker-code/CHANGELOG.md per the
sync-changelog skill.

Author: elkaix