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>

src/fabledassistant/mcp/server.py

@@ -36,7 +36,8 @@ Mechanics:
 - Tags are plain strings (no `#` prefix). Empty list clears tags; omit to leave
   unchanged on updates.
 - For optional integer FKs (project_id, milestone_id, parent_id), use 0 to mean
-  "not set".
+  "not set". On update_task, -1 clears an existing FK (e.g. milestone_id=-1
+  removes the task from its milestone); 0 leaves it unchanged.
 
 Keep task state honest — this is what makes the project a trustworthy record:
 - When you begin working a task, set it to in_progress (update_task

src/fabledassistant/mcp/tools/tasks.py

@@ -14,7 +14,7 @@ Sentinels (preserved from existing fable-mcp):
     what makes a Note a Task)
   - priority="none" sets explicit no-priority; priority="" is "leave unchanged"
   - project_id=0 / milestone_id=0 / parent_id=0 → "no association" on create,
-    "leave unchanged" on update
+    "leave unchanged" on update; on update, -1 clears the FK (sets it NULL)
 """
 from __future__ import annotations
 
@@ -147,8 +147,10 @@ async def update_task(
             the lifecycle: set in_progress when you start, done when complete —
             don't leave finished work at todo.
         priority: New priority — one of: none, low, medium, high.
-        project_id: New project. Omit (0) to leave unchanged.
-        milestone_id: New milestone. Omit (0) to leave unchanged.
+        project_id: New project. 0 = leave unchanged, -1 = clear (remove from
+            its project; also clears the milestone), positive = set.
+        milestone_id: New milestone. 0 = leave unchanged, -1 = clear (remove
+            from its milestone), positive = set.
     """
     uid = current_user_id()
     fields: dict = {}
@@ -160,9 +162,15 @@ async def update_task(
         fields["status"] = status
     if priority:
         fields["priority"] = priority
-    if project_id:
+    # Optional FKs: 0 = leave unchanged, -1 = clear (set NULL), positive = set.
+    if project_id == -1:
+        fields["project_id"] = None
+        fields["milestone_id"] = None  # a milestone can't outlive its project
+    elif project_id:
         fields["project_id"] = project_id
-    if milestone_id:
+    if milestone_id == -1:
+        fields["milestone_id"] = None
+    elif milestone_id:
         fields["milestone_id"] = milestone_id
     note = await notes_svc.update_note(uid, task_id, **fields)
     if note is None:

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()