Track running state of long task in a single dedicated file that survives compaction.
USE WHEN: task is long, agent has lost thread, user asks "where are we" / "到哪了" / "我们到哪了", before `context-pressure-compact`, `todowrite` alone is too thin, agent has done > 10 tool calls, "lost the thread" / "继续" / "忘了".
TRIGGER PHRASES: "where are we", "到哪了", "我们到哪了", "继续", "lost thread", "忘了", "lost the thread", "我们刚才说到哪了", "走神了", "回到主线".
SKIP WHEN: short task (<5 tool calls), single one-shot question, "do X" with X being small.
