Every cron job prompt now includes guidance that the agent can respond
with [SILENT] when it has nothing new or noteworthy to report. The
scheduler checks for this marker and skips delivery, while still saving
output to disk for audit. Failed jobs always deliver regardless.
This replaces the notify parameter approach from PR #1807 with a simpler
always-on design — the model is smart enough to decide when there's
nothing worth reporting without needing a per-job flag.
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