ENH: modernize package configuration

[redeboer]

• This fork will be published as PyPI package compwa-python-constraint 1.4.0.post1.
• Package metadata and setuptools configuration has been migrated from setup.py and setup.cfg to PEP 621 metadata in pyproject.toml.
• The package is now built with standards-compliant .tar.gz and universal wheel artifacts (PEP 625).
• Added a release-triggered trusted-publishing workflow used by ComPWA projects.

⚙️ Enhancements

• Package metadata is now declared in pyproject.toml (PEP 621) with setuptools>=77 as build backend; setup.py and setup.cfg are removed.
• The distribution name is now compwa-python-constraint, with Repository/Issues URLs pointing at ComPWA/python-constraint and Documentation still pointing at upstream.
• License metadata uses the SPDX form (license = "BSD-2-Clause" plus license-files) instead of the deprecated License :: classifier.
• Version is single-sourced from constraint.version.__version__ via [tool.setuptools.dynamic], and bumped to 1.4.0.post1 to mark the repackaged release.

🖱️ Developer experience

• CI (.github/workflows/ci.yml): runs the test suite on Python 3.10–3.14 via uv run --no-project, and a separate job that runs uv build, twine check, and then installs the built wheel into an isolated environment and runs tests/ against it — so packaged content is verified, not just the working tree.
• CD (.github/workflows/cd.yml): publishes to PyPI on release via trusted publishing (id-token: write), using the ComPWA reusable workflows.
• FORCE_COLOR is set in CI so test and build output is coloured in the GitHub Actions log.
• All GitHub Actions are pinned to commit SHAs, with Dependabot configured to update them yearly.
• Tombi is configured for pyproject.toml (4-space indent, 88-column width, uv.lock excluded), and the file is reordered into conventional PEP 621 key order.
• .gitignore now excludes agent configuration files (/.claude/, /.codex/, /.agents/, AGENTS.md, CLAUDE.md) as well as .venv/ and uv.lock.

📝 Documentation

• README.rst is converted to Markdown as README.md, so it renders natively on GitHub and PyPI without a reST toolchain; readme in pyproject.toml points at the new file.
• The README now opens with a "Why this fork exists" section: upstream 1.4.0 ships only a .tar.bz2 source distribution, which uv refuses since 0.12.0, so this fork republishes the same solver as a PEP 625 .tar.gz plus a py3-none-any wheel. It states explicitly that solver changes belong upstream.
• The stale Travis/Landscape/Coveralls badges are replaced by a CI badge for this repository.

Squash commit messages

* DOC: describe why this packaging fork exists
* DX: add Tombi configuration for `pyproject.toml`
* DX: ignore virtual environments and agent configuration
* DX: pin GitHub Actions to SHA and add Dependabot
* DX: publish to PyPI on release
* DX: run tests and build distributions on GitHub Actions
* ENH: show colors in CI logs
* MAINT: convert README from reST to Markdown