feat(steering): settle a steered turn at idle, not at the interrupt (#958)

Steering is delivered at priority `now`, which makes the CLI **abort**
the running cycle instead of folding the message into it. The aborted
cycle ends with an ordinary human-origin `result`, so we answered
`session/prompt` with `end_turn` while the steered work was still going
— its remaining tool calls, its answer, and its file edits then streamed
with no turn in flight.

A steered turn's results now only record their outcome, and the turn
settles at the SDK's `idle` — the only signal spanning both the
interrupted and the steered cycle. The injected message's replay tells
them apart: an idle arriving before it means the steered cycle hasn't
started, so it is swallowed. `session/cancel` and the next prompt still
settle the turn early, with the recorded stop reason.

Four steering tests cover the interrupt, an early idle, the next-prompt
hand-off, and cancel.

examples/simple-client.ts

@@ -29,9 +29,11 @@
  *   !steer <text>   `_session/steering` — injected into the turn that is
  *                   already running, so it can change course between tool
  *                   calls. Answers "injected", or "startedNewTurn" if the turn
- *                   beat us to the finish. Either way the reply arrives as
- *                   `session/update` notifications that can outlast the
- *                   `stopReason` of the turn you steered. See ./steering.ts.
+ *                   beat us to the finish. An injected reply streams as
+ *                   `session/update` notifications inside that turn, whose
+ *                   `stopReason` waits for it. "startedNewTurn" output belongs
+ *                   to a detached turn no `session/prompt` of ours tracks.
+ *                   See ./steering.ts.
  *   !queue <text>   `session/prompt` sent mid-turn anyway. The agent advertises
  *                   `promptQueueing`, so it takes the backlog instead of us —
  *                   same ordering, different owner.
@@ -564,8 +566,8 @@ async function main() {
     try {
       const result = await agent.request<SteeringResponse>(STEERING_METHOD, params);
       // Either way the steered message's own output arrives as `session/update`
-      // notifications, not in this response — and it may well keep streaming
-      // after the turn we steered has already answered with its stopReason.
+      // notifications, not in this response. When injected, it lands inside the
+      // turn we steered, whose `stopReason` waits for it.
       log(
         result.outcome === "injected"
           ? "steer outcome: injected into the running turn"