fix(db): auto-unassign done tickets on sprint close

sprint stop now clears assigned_to on all done tickets in the closing
sprint. Agents don't clean up after themselves — this was causing
5 bookkeeping warnings every sprint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tooling/db/sprint

@@ -369,6 +369,16 @@ def cmd_stop(args):
         done = done + auto_closed
         incomplete = [t for t in incomplete if t["status"] not in picked_up_statuses]
 
+    # Unassign all done tickets in this sprint (agents don't clean up after themselves)
+    conn = get_connection(load_config())
+    conn.execute(
+        "UPDATE tickets SET assigned_to=NULL WHERE sprint_id=? AND status='done' AND assigned_to IS NOT NULL",
+        (sprint["id"],)
+    )
+    unassigned = conn.total_changes
+    conn.commit()
+    conn.close()
+
     # Complete the sprint
     conn = get_connection(load_config())
     conn.execute(