goose's own steer path sets metadata.steer (agent.rs:540) and its ACP
surface republishes it as _meta.goose.steer. buzz-agent queued steers as
plain user messages, so nothing downstream could tell a mid-turn steer
apart from the user simply sending another message -- which is the one
distinction the flag exists to preserve.
buzz-acp does not read it today; this keeps the conversation record
honest for anything that later does.
Co-authored-by: Michael Neale <michael.neale@gmail.com>
Signed-off-by: Michael Neale <michael.neale@gmail.com>
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