Prepare v0.2.0 release + pin Action examples to tags (#31, #32)

[askmy-stack]

Summary

• Bump package to 0.2.0 and move Unreleased changelog notes into a dated release section
• README install now uses pip install tool-semantics (first PyPI cut)
• docs/github-action.md: pin compare Action to @v0.2.0, prefer PyPI install, add Versioning the Action note that @main may break
• Document release checklist extras in docs/publishing.md

Closes #31
Closes #32

Post-merge (needs owner)

1. Create GitHub Environment pypi (if missing)
2. Configure PyPI trusted publisher for askmy-stack/tool-semantics → workflow publish.yml, environment pypi (see docs/publishing.md)
3. Publish GitHub Release v0.2.0 from main to trigger publish.yml
4. Verify pip install tool-semantics==0.2.0 in a clean venv

Test plan

• pytest / ruff / mypy pass locally
• python -m build produces tool_semantics-0.2.0 sdist + wheel
• After trusted publisher + Release: PyPI publish workflow green
• pip install tool-semantics==0.2.0 works from clean venv

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