sweep: dedup session->Turn conversion, fix drifted line reference (V-581)

chatHistory (voice.go) and rememberTurn (clarify.go) both built the same
dialogue.Turn{Intent, Slots, Text} projection of a *dialogue.Session
inline; factor it into sessionAsTurn and use it in both. Also name the
history-depth cap (previously a bare "3") as maxCarriedHistory, and fix
chatHistory's doc comment, which cited "lines 373-395" for the dialogue
merge in runTurn -- that block has since moved to lines 385-394. Point
at the step-6 comment instead of a line range so the reference survives
future edits. No behavior change; bookkeeping only, not the clarify/
reminder slot-decision logic.

cmd/mavend/clarify.go

@@ -527,20 +527,22 @@ func (h *reactiveHandler) finishClarified(ctx context.Context, dec router.Decisi
 	return reply
 }
 
+// maxCarriedHistory — how many turns of PRIOR history (beyond the immediate
+// last turn) rememberTurn carries forward. The session ends up holding this
+// many plus the one just-finished turn, so callers describing the total
+// depth (chatHistory's doc comment, this one) say "up to 4".
+const maxCarriedHistory = 3
+
 // rememberTurn stores this turn as the dialogue session the next follow-up
 // inherits from, carrying up to 4 prior turns of history for anaphora. Capped so
 // one long conversation can't grow the session unboundedly.
 func (h *reactiveHandler) rememberTurn(ctx context.Context, prev *dialogue.Session, dec router.Decision, now time.Time) {
 	var history []dialogue.Turn
 	if prev != nil {
-		history = append(history, dialogue.Turn{
-			Intent: prev.Intent,
-			Slots:  prev.Slots,
-			Text:   prev.Slots.Text,
-		})
+		history = append(history, sessionAsTurn(prev))
 		maxHist := len(prev.History)
-		if maxHist > 3 {
-			maxHist = 3
+		if maxHist > maxCarriedHistory {
+			maxHist = maxCarriedHistory
 		}
 		history = append(history, prev.History[:maxHist]...)
 	}