why: Cut the next alpha covering the light-mode Pygments default and the
argparse-directive theme scoping fix — both shipped under #24 and visible
in the rendered docs as soon as a downstream project upgrades.

what:
- just bump-version 0.0.1a13: every workspace package's
  pyproject.toml version, every __init__.py / extension.py
  __version__, the workspace pyproject.toml, scripts/ci helpers
  and matching test, and uv.lock all advance to 0.0.1a13.
- CHANGES gains a dated "## gp-sphinx 0.0.1a13 (2026-04-30)"
  release header above the existing What's new / Bug fixes
  content; the "## gp-sphinx 0.0.1 (unreleased)" placeholder +
  maintainer comment stays at the top of the file as the next-
  release insertion point.