Move PyPI notifications to shared notification package

[robbykap]

Summary

Moves the PyPI notification migration out of #8 into its own stacked PR.

What changed

• adds a composite action for sending PyPI notifications
• migrates poetry_publish.yaml to use the shared notifications/ package instead of the Docker-based script
• adds notifications.send_pypi and PyPI formatting helpers
• removes the legacy pypi_updates/send_update_notification.py Docker path
• keeps the embed size coverage for the PyPI payload

Notes

• Base branch: codex-plain-text-notifications
• After Redesign Discord notification system with rich embeds #8 merges, this PR can be retargeted to main if needed.

Verified

• PYTHONPATH=. pytest -q tests/test_workflow_wiring.py
• python3 -m py_compile notifications/formatting/formatting_utils.py notifications/formatting/pypi_format.py notifications/send_pypi.py tests/test_embed_chunking.py