Prep v0.2.1 release (#490)

Bump workspace version to 0.2.1 and add changelog entry for the two
publishing fixes: shipping pre-compiled CSS in holt-book and publishing
holt-regression as a dependency of holt-cli.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Prep v0.2.0 release (#487)

Bump workspace version to 0.2.0, update holt-macros dependency version
in holt-book, and add CHANGELOG.md entries for both v0.1.0 and v0.2.0.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>