docs(agent): clarify late steering timing

.agents/notes/implemented/simplification/2026-07-17-one-send-one-turn.md

@@ -10,7 +10,7 @@ An ordinary `Agent.send()` payload is one complete caller message. Opportunistic
 
 An ordinary turn contains prompt admission, `turn/start`, `turn/end`, and the durability checkpoint. Combining messages would let a later ordinary message join an earlier message's model request instead of observing the earlier ordinary turn's closed result in the same session log, while mixed allowed and blocked prompts would require lifecycle states no caller explicitly requested.
 
-`steer()` already expresses joining the active turn, while `inject()` records model-facing context without acting as an ordinary message. Implicit ordinary-send batching would make `send()` overlap both explicit operations instead of preserving a single meaning.
+`steer()` already selects steering semantics while the agent driver is running, while `inject()` records model-facing context without acting as an ordinary message. Implicit ordinary-send batching would make `send()` overlap both explicit operations instead of preserving a single meaning.
 
 ## Decision
 
@@ -18,7 +18,7 @@ Each successful `send()` synchronously validates agent state, snapshots and free
 
 Prompt admission decides one message. An allowed prompt becomes that turn's `user/message`; a blocked prompt appends one durable `prompt/blocked` and ends that one-message turn as `rejected`. There are no mixed-batch or all-blocked-batch branches.
 
-Running `steer()` appends to the active turn's steering FIFO. Idle `steer()` delegates to `send()` and therefore creates an independent ordinary queue item. `inject()` retains its turn-enclosure and flush behavior. `cancel()`, `status`, and `whenIdle()` remain whole-agent operations rather than per-message controls.
+Running `steer()` adds to the steering FIFO. An open turn records it at the next steering checkpoint before a request or continuation decision. Steering can make continuation default to another step, but continuation or terminal policy can still stop before that step begins. After turn close and its durability checkpoint, remaining steering becomes later queued input. Terminal `agent/turn-stop`, cancellation, or disposal may discard it. Idle `steer()` delegates to `send()` and therefore creates an independent ordinary queue item. `inject()` retains its turn-enclosure and flush behavior. `cancel()`, `status`, and `whenIdle()` remain whole-agent operations rather than per-message controls.
 
 ## Alternatives considered
 
@@ -30,10 +30,10 @@ Running `steer()` appends to the active turn's steering FIFO. Idle `steer()` del
 - A real-composition test pipes two lines through the built stdio binary and observes two model requests and two turn boundaries.
 - A deferred first ordinary-turn flush proves the next queued ordinary turn cannot start before the checkpoint settles and that its request sees the preceding assistant result; a rejected flush still settles before the next ordinary turn starts.
 - Prompt veto and listener failure, broad cancellation, disposal, and pre-commit `turn/start` failure preserve balanced recorded turns and do not merge or strand surviving queued work.
-- Running and idle `steer()`, `inject()`, whole-agent status, and `whenIdle()` retain their existing coverage.
+- Open-turn, post-turn-close, and idle `steer()`, `inject()`, whole-agent status, and `whenIdle()` retain their existing coverage.
 
 ## Consequences
 
-Ordinary turn boundaries are deterministic, and a FIFO successor that reaches turn processing observes the preceding completed ordinary turn's closed session result after that turn's checkpoint settles; settlement does not mean a failed flush became durable. Several queued items can still run under one global `running` interval, and broad cancellation can discard the entire unstarted tail, so status and quiescence remain agent-wide observations rather than per-message results.
+Ordinary turn boundaries are deterministic, and a FIFO successor that reaches turn processing observes the preceding completed ordinary turn's closed session result after that turn's checkpoint settles; settlement does not mean a failed flush became durable. Several queued items can still run under one global `running` interval, which can also cover turn close and its checkpoint, so `running` does not prove a turn is open. Broad cancellation can discard the entire unstarted tail, and status and quiescence remain agent-wide observations rather than per-message results.
 
 Workloads that relied on coincidental ordinary-send batching make more model requests, incur more checkpoints, and may take longer to drain; FIFO queues may grow under sustained producers. Ordinary-send batching can return only through an explicit measured contract.