fix(chat): scope the steered-copy match to what the detail lists this round

Anchoring by position in the timeline still reached turns that are not
part of this round's projection: `localTurns` render as phase
"persisted" and a mid-turn refetch preserves them, so an earlier
round's promoted prompt sits after the anchor and a steer repeating its
words hid it. The candidates are now the detail's own user turns after
the in-flight prompt, matched by id — the only turns the agent can have
written during the running turn.