docs: implementation plan for tag-triggered PyPI CD

Five-task plan landed in commits 07b440c, b058986, ef861a2: pin
pyproject to 0.0.0-dev, write release.yml with OIDC Trusted
Publishing, rewrite RELEASING.md, document the one-time
PyPI/GitHub environment setup, push to main.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: markovejnovic