feat(onboarding): add provider rollout staging

[typelicious]

What changed

• add staged provider-rollout reporting to the onboarding report
• surface fallback-target readiness and image-provider gaps for many-provider rollouts
• extend onboarding docs and tests around staged provider introduction

Why

• make many-provider onboarding safer and easier to phase
• reduce the jump from simple readiness output to real rollout sequencing

How verified

• python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_onboarding.py tests/test_config.py tests/test_updates.py
• ./.venv-check-313/bin/ruff check foundrygate/onboarding.py tests/test_onboarding.py README.md docs/ONBOARDING.md CHANGELOG.md
• git diff --check