feat: cron agents can suppress delivery with [SILENT] response

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.

tests/cron/test_scheduler.py

@@ -7,7 +7,7 @@ from unittest.mock import AsyncMock, patch, MagicMock
 
 import pytest
 
-from cron.scheduler import _resolve_origin, _resolve_delivery_target, _deliver_result, run_job
+from cron.scheduler import _resolve_origin, _resolve_delivery_target, _deliver_result, run_job, SILENT_MARKER
 
 
 class TestResolveOrigin:
@@ -449,3 +449,97 @@ class TestRunJobSkillBacked:
         assert "Instructions for blogwatcher." in prompt_arg
         assert "Instructions for find-nearby." in prompt_arg
         assert "Combine the results." in prompt_arg
+
+
+class TestSilentDelivery:
+    """Verify that [SILENT] responses suppress delivery while still saving output."""
+
+    def _make_job(self):
+        return {
+            "id": "monitor-job",
+            "name": "monitor",
+            "deliver": "origin",
+            "origin": {"platform": "telegram", "chat_id": "123"},
+        }
+
+    def test_normal_response_delivers(self):
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(True, "# output", "Results here", None)), \
+             patch("cron.scheduler.save_job_output", return_value="/tmp/out.md"), \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            from cron.scheduler import tick
+            tick(verbose=False)
+        deliver_mock.assert_called_once()
+
+    def test_silent_response_suppresses_delivery(self, caplog):
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(True, "# output", "[SILENT]", None)), \
+             patch("cron.scheduler.save_job_output", return_value="/tmp/out.md"), \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            from cron.scheduler import tick
+            with caplog.at_level(logging.INFO, logger="cron.scheduler"):
+                tick(verbose=False)
+        deliver_mock.assert_not_called()
+        assert any(SILENT_MARKER in r.message for r in caplog.records)
+
+    def test_silent_with_note_suppresses_delivery(self):
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(True, "# output", "[SILENT] No changes detected", None)), \
+             patch("cron.scheduler.save_job_output", return_value="/tmp/out.md"), \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            from cron.scheduler import tick
+            tick(verbose=False)
+        deliver_mock.assert_not_called()
+
+    def test_silent_is_case_insensitive(self):
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(True, "# output", "[silent] nothing new", None)), \
+             patch("cron.scheduler.save_job_output", return_value="/tmp/out.md"), \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            from cron.scheduler import tick
+            tick(verbose=False)
+        deliver_mock.assert_not_called()
+
+    def test_failed_job_always_delivers(self):
+        """Failed jobs deliver regardless of [SILENT] in output."""
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(False, "# output", "", "some error")), \
+             patch("cron.scheduler.save_job_output", return_value="/tmp/out.md"), \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            from cron.scheduler import tick
+            tick(verbose=False)
+        deliver_mock.assert_called_once()
+
+    def test_output_saved_even_when_delivery_suppressed(self):
+        with patch("cron.scheduler.get_due_jobs", return_value=[self._make_job()]), \
+             patch("cron.scheduler.run_job", return_value=(True, "# full output", "[SILENT]", None)), \
+             patch("cron.scheduler.save_job_output") as save_mock, \
+             patch("cron.scheduler._deliver_result") as deliver_mock, \
+             patch("cron.scheduler.mark_job_run"):
+            save_mock.return_value = "/tmp/out.md"
+            from cron.scheduler import tick
+            tick(verbose=False)
+        save_mock.assert_called_once_with("monitor-job", "# full output")
+        deliver_mock.assert_not_called()
+
+
+class TestBuildJobPromptSilentHint:
+    """Verify _build_job_prompt always injects [SILENT] guidance."""
+
+    def test_hint_always_present(self):
+        from cron.scheduler import _build_job_prompt
+        job = {"prompt": "Check for updates"}
+        result = _build_job_prompt(job)
+        assert "[SILENT]" in result
+        assert "Check for updates" in result
+
+    def test_hint_present_even_without_prompt(self):
+        from cron.scheduler import _build_job_prompt
+        job = {"prompt": ""}
+        result = _build_job_prompt(job)
+        assert "[SILENT]" in result