ci: enable PyPI trusted publishing

[Chris1221]

Summary

• Uncomments the pypa/gh-action-pypi-publish step in the publish workflow, enabling automatic PyPI releases when a GitHub release is created by release-please
• Adds the build package as a PyPI dependency so pixi run python -m build works in CI
• Uses OIDC trusted publishing (no API token needed)

Before merging

Configure the trusted publisher on PyPI at https://pypi.org/manage/project/yamldoc/settings/publishing/ with:

• Owner: Chris1221
• Repository: yamldoc
• Workflow: python-publish.yml
• Environment: pypi

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (9082c42) to head (41d00b8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   90.17%   90.17%           
=======================================
  Files           7        7           
  Lines         641      641           
=======================================
  Hits          578      578           
  Misses         63       63           

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