fix(ops_run): unique triggering_event_id per cron fire

Cron schedules always passed trigger_key="scheduled", so ops_run
idempotency collapsed every weekday into one key. After the first fire,
create_ops_run was a silent no-op, the claim loop starved, and dual-clock
host timers produced empty FI briefs.

Map scheduled fires to run_id (or scheduled:{iso}) via
emit_triggering_event_id; log duplicate skips; document the contract.

tests/test_ops_run_queue.py

@@ -11,6 +11,7 @@ import pytest
 
 from activity_core.ops_run_queue import (
     build_idempotency_key,
+    emit_triggering_event_id,
     max_attempts,
     ops_run_queue_enabled,
     ops_run_to_dict,
@@ -39,6 +40,58 @@ def test_build_idempotency_key() -> None:
     )
 
 
+def test_emit_triggering_event_id_scheduled_uses_run_id() -> None:
+    """Bare 'scheduled' must not be the ops_run key for every weekday fire."""
+    day1 = emit_triggering_event_id("scheduled", "run-day-1")
+    day2 = emit_triggering_event_id("scheduled", "run-day-2")
+    assert day1 == "run-day-1"
+    assert day2 == "run-day-2"
+    assert day1 != day2
+
+
+def test_emit_triggering_event_id_prefers_scheduled_for() -> None:
+    got = emit_triggering_event_id(
+        "scheduled",
+        "run-fallback",
+        scheduled_for="2026-08-05T05:30:00+00:00",
+    )
+    assert got == "scheduled:2026-08-05T05:30:00+00:00"
+
+
+def test_emit_triggering_event_id_passes_through_events_and_manual() -> None:
+    assert emit_triggering_event_id("evt-abc", "run-x") == "evt-abc"
+    assert (
+        emit_triggering_event_id("manual-3da4cf06", "run-y") == "manual-3da4cf06"
+    )
+
+
+def test_scheduled_fires_produce_distinct_ops_keys() -> None:
+    """Regression: two cron days must not share one ops_run idempotency key."""
+    def_id = "3169ab1f-882b-59c7-9763-d014dc96f4fc"
+    source = "emit-fi-daily-brief-task"
+    key_aug4 = build_idempotency_key(
+        TaskSpec(
+            title="FI",
+            activity_definition_id=def_id,
+            source_id=source,
+            triggering_event_id=emit_triggering_event_id("scheduled", "run-aug4"),
+        )
+    )
+    key_aug5 = build_idempotency_key(
+        TaskSpec(
+            title="FI",
+            activity_definition_id=def_id,
+            source_id=source,
+            triggering_event_id=emit_triggering_event_id("scheduled", "run-aug5"),
+        )
+    )
+    # Old bug: both would be "...:scheduled"
+    legacy = f"{def_id}:{source}:scheduled"
+    assert key_aug4 != key_aug5
+    assert key_aug4 != legacy
+    assert key_aug5 != legacy
+
+
 def test_max_attempts(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.delenv("OPS_RUN_MAX_ATTEMPTS", raising=False)
     assert max_attempts() == 3