Companion to gateway-gateway#160 (connector absent-row default flips to
mention-gated / requireAddress=true).
relay_relevance_policy() previously returned None whenever the projected
policy was all-falsy, on the premise that 'the connector's quiet default
already matches'. Once the connector defaults requireAddress to true,
that premise inverts for one case: an agent EXPLICITLY configured with
require_mention: false would stay silent and get mention-gated anyway,
with no way out.
The nothing-to-declare check now keys on 'require_mention is unset'
rather than 'require_mention is falsy': an explicit false IS a
configured knob and is declared to the connector; a genuinely
unconfigured platform still declares nothing and inherits the
connector's default. No wire/contract change.
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