Model router resumes a quota-halted turn with a synthetic user message

pi has no extension API to re-run a failed turn: the only way an extension can start agent work again is pi.sendUserMessage(), which appends a real user message to the transcript. The Model router therefore sends one fixed Recovery message after a wait or a model switch, telling the model that the previous turn was cut off by a usage limit and to resume the task.

Rejected alternatives: provider-level retry, because pi's own settings docs warn it can block the agent until the quota resets; manual re-trigger, because it defeats the unattended workflow the router exists for; and waiting on an upstream retry-turn API, which does not exist today. One visible user message per recovery in the transcript is the accepted cost.