Runs

A run is one coding task Syndai carries from a prompt to a delivered change. Creating a run starts billable work; its `external_request_id` is the idempotency key, so a retry never starts a second run. Follow a run by streaming its events or polling `GET /runs/{id}`. Phases are open-ended: a run can pause to ask you something, so read `pending_hitl_requests` and answer through `/responses` rather than assuming a fixed order.