Refactor CI into composite actions and add release/publish workflows

[blink1073]

References

Description

Refactors existing CI jobs into reusable composite actions and introduces new release and publish workflows. The goal is to make these actions easy to extract into a dedicated repository under the calysto org that can be shared across projects. The actions assume the use of "uv" (and optionally "just", where the justfile, if given, has an "install" target).

Changes

• Move version from metakernel/__init__.py to a static field in pyproject.toml; use importlib.metadata.version() in __init__.py
• Add .github/actions/release/ — composite action that bumps version, updates changelog, creates a GitHub release, and bumps to dev
• Add .github/actions/publish/ — composite action that builds, inspects, and publishes the package to PyPI
• Add .github/actions/enforce-label/ — composite action wrapping the enforce-label logic
• Add .github/actions/test-sdist/ — composite action that downloads and tests the sdist artifact
• Add .github/workflows/release.yml — triggers release action on workflow_dispatch or daily cron (dry run)
• Add .github/workflows/publish.yml — builds and publishes to Test PyPI (on release) and PyPI (on release), skipped on forks
• Update enforce-label.yml and tests.yml to call the new composite actions
• Remove legacy prep-release.yml, publish-changelog.yml, publish-release.yml

Backwards-incompatible changes

None

Testing

N/A

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: Claude Sonnet 4.6 (Claude Code)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (f6cf010) to head (5b240e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #395   +/-   ##
=======================================
  Coverage   90.28%   90.29%           
=======================================
  Files          51       51           
  Lines        2945     2946    +1     
  Branches      414      414           
=======================================
+ Hits         2659     2660    +1     
  Misses        209      209           
  Partials       77       77           

Files with missing lines	Coverage Δ
metakernel/__init__.py	91.66% <100.00%> (+0.75%)	⬆️

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