v0.4.0 — every row says why

Every action row now carries the reason for that step.

The Host fold keeps the sentence the model wrote right before a tool call and attaches it to the
row; the card renders it as 为了 … (or "for …" in English). When there is no such sentence —
a long scripted chain, or a model that called the tool with no preface — the card falls back to the
todo item the agent is currently working on, so "跑 Python 脚本 · patch.py" no longer stands alone.

• why on each item and on each queued plan call, capped at 80 characters.
• stateVersion 6: existing sessions refold and gain reasons retroactively.
• Host-side change, so one Harness restart is needed; the card layout itself already reloads with a
  page refresh.