Tell the model when a message was a steer

A message typed during a turn reaches the model at the next model call if
the turn has one left, and otherwise as the opening line of the next turn --
Claude's read out of the fifo after the turn ended, Codex's requeued when
turn/steer is refused. Read there it is indistinguishable from a reply, so
the model treats the answer it just gave as seen.

Both drivers now compose the text the CLI receives through
driver::message_body, which prefixes a note saying the message was written
without having seen the rest of that turn. The transcript still holds the
words that were typed; only the CLI's copy carries the note.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PLAN.md

@@ -453,6 +453,27 @@ error row a screen away.
 `Unknown` is not "we could not find out": a driver that is gone reported
 everything it was holding when it closed.
 
+### A steer says that it is one (2026-09-13)
+
+A message typed during a turn reaches the model with a bracketed note in
+front of it saying it was written without having seen the rest of that turn
+(`driver::message_body`, and `STEERING_NOTE` beside it). The transcript keeps
+the words that were typed; only the copy the CLI is handed carries the note.
+
+The reason is that where a steer lands is not ours to choose. It reaches the
+model at the next model call if the turn has one left, and otherwise as the
+opening line of the *next* turn -- Claude's read out of the fifo after the
+turn ended, Codex's requeued when `turn/steer` is refused as
+`activeTurnNotSteerable`. In that second case nothing distinguishes it from
+an ordinary reply, so the model treats the answer it just gave as read and
+answers around it. Bryan reported this as the ordinary experience of steering
+from the phone: the interruption is meant to arrive mid-work, and it lands
+after the fact often enough to matter.
+
+It is prefixed on every steer rather than only on the ones that land late,
+because the two are the same message until the CLI reads it, and the note is
+true either way: a steer never saw the rest of the turn it was typed into.
+
 ### Session processes outlive the backend (2026-08-29)
 
 A session's process is **left running when the backend stops and adopted