Cut first GitHub Release + PyPI publish; update README install

[askmy-stack]

Summary

`publish.yml` and trusted-publishing docs exist (#23), but there are no GitHub Releases yet. README still says `# later: pip install tool-semantics`.

Acceptance criteria

• One-time PyPI trusted publisher + `pypi` GitHub Environment configured (see `docs/publishing.md`)
• Version bumped/consistent in `pyproject.toml` and `src/tool_semantics/init.py`
• CHANGELOG: move Unreleased → a dated version section (e.g. `0.1.0` or `0.2.0` if features warrant)
• Tag + GitHub Release `vX.Y.Z` triggers successful publish workflow
• README install section uses `pip install tool-semantics` (remove "later")
• Verify `pip install tool-semantics` works from a clean venv

Why

Without a published package, the composite Action's `pip install tool-semantics` fallback and library adoption are blocked.