ci: add PyPI publishing on tag push

[ericchansen]

Summary

Set up automated PyPI publishing so that pushing a git tag (v5.0.0) automatically builds and publishes the package to PyPI.

Changes

New: .github/workflows/publish.yml

• Triggers on v* tag pushes
• Builds sdist + wheel using python -m build
• Verifies the package version matches the git tag
• Publishes via PyPI trusted publishing (OIDC -- no API tokens stored)
• Uses a pypi environment for deployment protection

Updated: pyproject.toml

• Dynamic versioning: Switched from static version = "5.0.0" to setuptools-scm which derives the version from git tags automatically
• PyPI metadata: Added readme, keywords, classifiers for a proper PyPI listing
• Dev deps: Added build to dev extras

How it works

# Tag a release
git tag v5.0.0
git push --tags

# GitHub Actions automatically:
# 1. Builds sdist + wheel (version = 5.0.0 from tag)
# 2. Verifies version matches tag
# 3. Publishes to PyPI via OIDC trusted publisher

Users can then install with:

pip install q2mm
pip install q2mm[openmm]    # with OpenMM backend
pip install q2mm[all]       # everything

One-time setup required

Before the first publish, register a pending publisher on PyPI:

1. Go to https://pypi.org/manage/account/publishing/
2. Add a pending publisher:
   • PyPI project name: q2mm
   • Owner: ericchansen
   • Repository: q2mm
   • Workflow name: publish.yml
   • Environment name: pypi
3. Create a pypi environment in GitHub repo settings (Settings > Environments > New)

Verified locally

• python -m build produces q2mm-4.1.dev55.tar.gz and .whl successfully
• setuptools-scm correctly derives version from git tags
• Ruff lint clean

[Copilot]

Pull request overview

Adds a GitHub Actions release pipeline to publish q2mm to PyPI on v* tag pushes, and updates packaging metadata to support tag-derived versions.

Changes:

• Introduces .github/workflows/publish.yml to build sdist/wheel on tag push and publish to PyPI via OIDC trusted publishing.
• Switches pyproject.toml to dynamic versioning using setuptools-scm and expands PyPI metadata (readme/keywords/classifiers).
• Adds build to the dev optional dependency group.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pyproject.toml	Enables setuptools-scm dynamic versioning and improves PyPI metadata/extras.
.github/workflows/publish.yml	Adds tag-triggered build + version check + PyPI publish workflow using OIDC.

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