diff: warn when --fail-on-broke cannot gate (governing engine unresolved)

[amis92]

Final M3 review F5: with a custom --engines list disjoint from --governing precedence, governing resolves to null and --fail-on-broke silently doesn't gate. Defaults make this unreachable in the shipped Action; emit a warning (and consider exit 2) when the flag is set but no governor resolved.

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https://claude.ai/code/session_01KcYK8hmTmXi8eLWpg9LGKJ