Get Run Status

Lightweight run status for progress polling. Returns only the volatile, frequently-changing fields: `status`, step counts (`total_steps`, `finished_steps`), timestamps (`started_at`, `completed_at`, `paused_at`, `pause_requested_at`), `usage_summary`, `error`, and `has_export`. The definition snapshot is intentionally omitted to keep response size small. Recommended polling pattern: call `GET /runs/{run_id}` once after starting a run to load the immutable definition snapshot and initial metadata, then poll this endpoint until `status` reaches a terminal value (`completed`, `failed`, or `cancelled`). `has_export` becoming `true` signals that a download is ready via `GET /runs/{run_id}/download`. The transient `pausing` state is observable here: `status == "running"` with `pause_requested_at` set means a pause is in progress but the current wave has not yet finished draining.