feat(ops): add rollout guardrails

[typelicious]

Supersedes #46 without force-pushing the original branch.

What changed

• adds provider-health rollout guardrails to the auto-update eligibility path
• adds config fields for require_healthy_providers and max_unhealthy_providers
• updates README, publishing, troubleshooting, and tests for the stricter rollout policy

Why

• prevents helper-driven updates from being marked eligible while the gateway is already degraded
• gives operators a small but meaningful safety budget for scheduled rollouts

How verified

• python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_updates.py tests/test_config.py
• ./.venv-check-313/bin/ruff check foundrygate/config.py foundrygate/main.py foundrygate/updates.py tests/test_updates.py tests/test_config.py
• ./.venv-check-313/bin/ruff format --check foundrygate/config.py foundrygate/main.py foundrygate/updates.py tests/test_updates.py tests/test_config.py
• bash -n scripts/foundrygate-auto-update
• /usr/bin/git diff --check