docs(onboarding): add client integration quickstarts

[typelicious]

What changed

• add built-in OpenClaw, n8n, and CLI quickstart hints to the onboarding report
• expand onboarding and integrations docs with copy/paste client examples
• cover the new onboarding examples in tests

Why

• make the v0.8 many-provider / many-client onboarding path more concrete for first-time operators
• reduce the gap between generic readiness output and real client setup steps

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 docs/INTEGRATIONS.md CHANGELOG.md
• ./.venv-check-313/bin/ruff format --check foundrygate/onboarding.py tests/test_onboarding.py
• git diff --check