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>