feat(coord): add due_at field to coord_todos

Migration 20260526_150000 adds nullable due_at datetime column. Model, schemas
(create/update/response), and sync.sh display updated. Sync output now shows
due:YYYY-MM-DDTHH:MM alongside any todo with a due date.

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

.claude/scripts/sync.sh

@@ -482,9 +482,11 @@ for group_name in sorted(groups.keys(), key=lambda x: (x == "Personal", x)):
         if t.get("assigned_to_machine") and t["assigned_to_machine"].upper() != "$TODO_MACHINE".upper():
             assigned += f"/{t['assigned_to_machine']}"
         auto = " [auto]" if t.get("auto_created") else ""
-        print(f"    [ ] {t['text']}{auto}{assigned}  (id:{t['id'][:8]})")
+        due = f"  due:{t['due_at'][:16]}" if t.get("due_at") else ""
+        print(f"    [ ] {t['text']}{auto}{due}{assigned}  (id:{t['id'][:8]})")
         for st in subtask_map.get(t["id"], []):
-            print(f"        [ ] {st['text']}  (id:{st['id'][:8]})")
+            st_due = f"  due:{st['due_at'][:16]}" if st.get("due_at") else ""
+            print(f"        [ ] {st['text']}{st_due}  (id:{st['id'][:8]})")
 PYEOF
         echo ""
     else