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