feat(onboarding): add client matrix reporting

[typelicious]

What changed

• add a client matrix to the onboarding report
• show which profiles come from presets vs custom config, how they match traffic, and what routing intent they apply
• warn when non-default client profiles have no real match rule

Why

• make many-client onboarding easier to reason about before traffic goes live
• reduce hidden profile behavior and unreachable client config

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
• ./.venv-check-313/bin/ruff format --check foundrygate/onboarding.py tests/test_onboarding.py