TOP = last element only for pop; multi-push puts first title on top for DFS
breakdown. Dropped nested-frame model; migrate old frames JSON to flat stack.
push of one or more titles opens a new frame (siblings at one depth);
pop leaves the frame. Supports T1,T2 → T1.1,T1.2 → pop → T2.1 pattern.
Legacy flat session JSON migrates to a single root frame.
Sub-task stack stored on session.todo_stack; model tools todo_*; slash
/todos for human ops; agent loop injects a review user message when open
todos exist and no todo_* tool ran this turn. Stack carries across /compact.