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_choreographer_completion_guards.py

@@ -12,6 +12,7 @@ These tests verify:
 
 from __future__ import annotations
 
+from datetime import UTC, datetime
 from typing import Any
 from unittest.mock import AsyncMock, MagicMock
 from uuid import uuid4
@@ -55,6 +56,13 @@ def _make_deps(**overrides: Any) -> ChoreographerDeps:
         "journal_highlights_for_task",
     ):
         getattr(repo, method).return_value = []
+    # C8: default-fresh journal:decision so PM-decision gate passes.
+    # Tests that exercise the gate boundary stub their own value.
+    # The check matches MagicMock and AsyncMock (the two default sentinel
+    # types pytest's unittest.mock leaves on un-stubbed return_values).
+    _ldef = base["journal"].latest_decision_at.return_value
+    if type(_ldef).__name__ in ("MagicMock", "AsyncMock"):
+        base["journal"].latest_decision_at.return_value = datetime.now(UTC)
     return ChoreographerDeps(**base)
 
 
@@ -83,6 +91,7 @@ async def test_cell_pm_complete_blocks_when_subtask_pending() -> None:
     task_svc.get_subtasks.return_value = [sub]
     journal_svc = AsyncMock()
     journal_svc.has_decision_for_task.return_value = True
+    journal_svc.latest_decision_at.return_value = datetime.now(UTC)
     journal_svc.has_reflect_for_task.return_value = True
     deps = _make_deps(task=task_svc, journal=journal_svc)
     c = Choreographer(deps)
@@ -118,6 +127,7 @@ async def test_cell_pm_complete_allows_when_all_terminal() -> None:
     task_svc.cell_pm_complete.return_value = after
     journal_svc = AsyncMock()
     journal_svc.has_decision_for_task.return_value = True
+    journal_svc.latest_decision_at.return_value = datetime.now(UTC)
     journal_svc.has_reflect_for_task.return_value = True
     git_svc = AsyncMock()
     git_svc.pr_merge.return_value = {"merge_commit_sha": "abc"}
@@ -155,6 +165,7 @@ async def test_main_pm_complete_blocks_when_subtask_pending() -> None:
     task_svc.get_subtasks.return_value = [sub]
     journal_svc = AsyncMock()
     journal_svc.has_decision_for_task.return_value = True
+    journal_svc.latest_decision_at.return_value = datetime.now(UTC)
     journal_svc.has_reflect_for_task.return_value = True
     deps = _make_deps(task=task_svc, journal=journal_svc)
     c = Choreographer(deps)
@@ -193,6 +204,7 @@ async def test_submit_up_blocks_when_subtask_pending() -> None:
     task_svc.get_subtasks.return_value = [sub]
     journal_svc = AsyncMock()
     journal_svc.has_decision_for_task.return_value = True
+    journal_svc.latest_decision_at.return_value = datetime.now(UTC)
     deps = _make_deps(task=task_svc, journal=journal_svc)
     c = Choreographer(deps)