feat(mcp): milestone_id=-1 clears a task's milestone (update_task)

Optional FKs on update_task previously had no way to express 'remove' —
0 meant leave-unchanged and any positive int meant set, so a milestone
(or project) could only be cleared via the web UI. Now -1 clears the FK
(NULL); clearing project_id also clears milestone_id since a milestone
can't outlive its project. update_note already NULLs on None, so the
change is confined to the tool wrapper. Closes scribe task #586.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_mcp_tool_tasks.py

@@ -164,6 +164,46 @@ async def test_update_task_raises_when_not_found():
             await update_task(task_id=999, status="done")
 
 
+@pytest.mark.asyncio
+async def test_update_task_milestone_zero_is_omitted():
+    """milestone_id=0 is 'leave unchanged' — must not reach the service."""
+    fake = _fake_task()
+    mock = AsyncMock(return_value=fake)
+    with patch("fabledassistant.mcp.tools.tasks.notes_svc.update_note", mock):
+        await update_task(task_id=1, milestone_id=0)
+    assert "milestone_id" not in mock.call_args.kwargs
+
+
+@pytest.mark.asyncio
+async def test_update_task_milestone_positive_is_set():
+    fake = _fake_task()
+    mock = AsyncMock(return_value=fake)
+    with patch("fabledassistant.mcp.tools.tasks.notes_svc.update_note", mock):
+        await update_task(task_id=1, milestone_id=42)
+    assert mock.call_args.kwargs["milestone_id"] == 42
+
+
+@pytest.mark.asyncio
+async def test_update_task_milestone_negative_one_clears():
+    """milestone_id=-1 clears the milestone (sets the column NULL)."""
+    fake = _fake_task()
+    mock = AsyncMock(return_value=fake)
+    with patch("fabledassistant.mcp.tools.tasks.notes_svc.update_note", mock):
+        await update_task(task_id=1, milestone_id=-1)
+    assert mock.call_args.kwargs["milestone_id"] is None
+
+
+@pytest.mark.asyncio
+async def test_update_task_clearing_project_also_clears_milestone():
+    """project_id=-1 clears the project and, with it, the milestone."""
+    fake = _fake_task()
+    mock = AsyncMock(return_value=fake)
+    with patch("fabledassistant.mcp.tools.tasks.notes_svc.update_note", mock):
+        await update_task(task_id=1, project_id=-1)
+    assert mock.call_args.kwargs["project_id"] is None
+    assert mock.call_args.kwargs["milestone_id"] is None
+
+
 @pytest.mark.asyncio
 async def test_add_task_log_returns_log_dict():
     log = MagicMock()