docs: sync v0.2.1 changelog with PyPI workflow

[CoderDeltaLAN]

Summary:

• Updates the v0.2.1 changelog with the PyPI Trusted Publishing workflow changes from PR ci: add PyPI trusted publishing workflow #73 and PR ci: split PyPI build and publish jobs #74.

Validation:

• Read CHANGELOG.md and publish-pypi.yml before editing.
• Changed only CHANGELOG.md.
• Local checks passed: 118 tests, Ruff, text hygiene, Git whitespace.
• Pre-push scope check passed.

Release note:

• This documents the release workflow only. It does not publish PyPI, create a tag, or create a GitHub Release.