Migrate to pyproject.toml and towncrier fragments

[MaxGhenis]

Summary

• Migrates from setup.py to pyproject.toml (hatchling build backend)
• Replaces changelog_entry.yaml with towncrier fragments (changelog.d/<branch>.<type>.md) to eliminate merge conflicts
• Each PR gets its own uniquely-named fragment file instead of all editing the same file
• Simplifies PR CI: lightweight check that a fragment exists (no Python install needed)
• Push workflow uses towncrier for version bumping and changelog generation

Fragment format

echo "Description." > changelog.d/<branch-name>.<type>.md
# Types: added (minor), changed (patch), fixed (patch), removed (minor), breaking (major)

🤖 Generated with Claude Code