Remind the current signer

Appends participant.reminder_sent for the current participant (lowest orderIndex still pending) and emits the notification event. Only valid while the envelope is in_progress. Rate-limited per participant (default: one reminder per hour) — exceeding the window returns 429. Supports the Idempotency-Key header: a replayed completed key returns the envelope's current graph with the original status code and Idempotency-Replayed: true (never re-executing); an in-flight duplicate is rejected 409; reusing a key with a different request is rejected 422.