feat(briefing): add task change detection helpers and task_id to _gather_internal

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_briefing_pipeline.py

@@ -29,6 +29,42 @@ def test_slot_greeting():
     assert slot_greeting("midday") != slot_greeting("afternoon")
 
 
+def test_compute_task_snapshot_hash():
+    """compute_task_hash should return a stable SHA-256 hex string."""
+    from fabledassistant.services.briefing_pipeline import compute_task_hash
+
+    task = {"status": "todo", "priority": "high", "due_date": "2026-03-25", "title": "Write spec"}
+    h = compute_task_hash(task)
+    assert len(h) == 64  # SHA-256 hex
+    assert h == compute_task_hash(task)
+    assert h != compute_task_hash({**task, "status": "done"})
+
+
+@pytest.mark.asyncio
+async def test_split_changed_tasks_all_new():
+    """split_changed_tasks should return all tasks as changed when no snapshot exists."""
+    from fabledassistant.services.briefing_pipeline import split_changed_tasks
+
+    tasks = [
+        {"task_id": 1, "title": "A", "status": "todo", "priority": "none", "due_date": None},
+    ]
+
+    with patch(
+        "fabledassistant.services.briefing_pipeline.async_session"
+    ) as mock_cls:
+        mock_session = AsyncMock()
+        mock_session.__aenter__ = AsyncMock(return_value=mock_session)
+        mock_session.__aexit__ = AsyncMock(return_value=False)
+        mock_session.execute = AsyncMock(return_value=MagicMock(
+            scalars=MagicMock(return_value=MagicMock(all=MagicMock(return_value=[])))
+        ))
+        mock_cls.return_value = mock_session
+
+        changed, unchanged_count = await split_changed_tasks(user_id=1, tasks=tasks)
+        assert len(changed) == 1
+        assert unchanged_count == 0
+
+
 @pytest.mark.asyncio
 async def test_post_message_accepts_metadata():
     """post_message should accept an optional metadata dict and store it."""