docs(release): prepare v0.8.0

[typelicious]

What changed

• bump runtime and package version from 0.7.0 to 0.8.0
• cut the onboarding work from Unreleased into the v0.8.0 changelog section
• update release guidance to include the v0.8.0 onboarding baseline

Why

• finalize the many-provider and many-client onboarding line as a releasable milestone
• prepare the repo for the v0.8.0 tag and GitHub release

How verified

• PYTHONPYCACHEPREFIX="$PWD/.pycache" python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q
• ./.venv-check-313/bin/ruff check .
• ./.venv-check-313/bin/python -m build --no-isolation
• ./.venv-check-313/bin/python -m twine check dist/foundrygate-0.8.0.tar.gz dist/foundrygate-0.8.0-py3-none-any.whl
• git diff --check