`buildAttentionInbox` previously surfaced every `message_read` sibling
the moment it was written, which meant the sender's preface lit up with
"B read your message — no reply yet" 30 seconds after B opened the
message. That's noise: B is almost certainly still composing a reply.

New `read_receipt_min_age_ms` option (default 5 minutes) suppresses
receipts younger than the threshold. Tests and hot-debug paths can
pass `read_receipt_min_age_ms: 0` to opt out. The existing receipt
test does this; a new ripening test asserts the gate behavior using
the existing `now` injection point so the assertion doesn't race the
wall clock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>