feat(gateway): C8 PM-decision gate windowed satisfaction

_check_pm_decision_required now requires the latest journal:decision
within pm_decision_window_seconds (default 300). Older decisions no
longer satisfy the gate. Adds JournalService.latest_decision_at.

Future-tighten (out of scope): per-verb-group consumption tracking
would need persistent state — Choreographer is per-request today.

tests/unit/gateway/test_heartbeat_wired.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from datetime import UTC, datetime
 from unittest.mock import AsyncMock, MagicMock
 from uuid import uuid4
 
@@ -126,6 +127,7 @@ async def test_i_am_done_calls_heartbeat() -> None:
     journal_svc.has_reflect_for_task.return_value = True
     # JOURNAL_DURING_WORK_AT_LEAST_ONE: ≥1 decision/learning/struggle entry.
     journal_svc.has_decision_for_task.return_value = True
+    journal_svc.latest_decision_at.return_value = datetime.now(UTC)
     journal_svc.has_learning_for_task.return_value = False
     journal_svc.has_struggle_for_task.return_value = False
     evidence_repo = AsyncMock()