feat(harness): per-run auto-approval of layer-2 proposals for model measurement

런 옵션 auto_approve_after_s(runs 열·POST /runs)를 추가한다. 값이 양수면 하네스가 제안
등록 뒤 그 지연만큼 기다렸다가 스스로 승인(actor auto)하고 HITL 버튼 경로와 같은 방식으로
조치를 실행(actor ai-approved, MarkProposalExecuted)한다. 사람 없이 모델 비교 캠페인을
반복하기 위한 측정 모드로, MTTA는 지연값으로 고정되고 판정 로직은 그대로다(2층 판정은
ai-approved 조치에서 나온다). 이미 결정·만료된 제안과 기록 전용 조치는 API 경로와 같이
처리한다.

harness/autoapprove.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"context"
+	"time"
+)
+
+// autoApprove is the measurement-mode stand-in for the Discord approver (run option auto_approve_after_s, 2026-10-15):
+// after the configured delay the harness approves the layer-2 proposal (actor "auto") and executes it exactly as the
+// HITL bot does after a button press (actor "ai-approved", MarkProposalExecuted). MTTA therefore equals the delay;
+// the verdict logic is unchanged (layer 2 from the ai-approved action). A proposal already decided, expired or
+// recorded-only is handled the same way as the API path.
+func (a *App) autoApprove(token string, req ProposalRequest, after time.Duration) {
+	time.Sleep(after)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Minute)
+	defer cancel()
+	p, err := a.store.GetProposal(ctx, token)
+	if err != nil || p.ApprovedAt != nil || time.Now().After(p.ExpiresAt) {
+		return
+	}
+	if err := a.store.ApproveProposal(ctx, token, "auto"); err != nil {
+		return
+	}
+	suffix := token[len(token)-6:]
+	a.store.Event(ctx, p.RunID, p.ScenarioID, "approval", "auto", map[string]any{"action_id": p.ActionID, "token_suffix": suffix, "auto_approve_after_s": after.Seconds()})
+	ce := catalogEntry(p.ActionID)
+	if ce == nil {
+		return
+	}
+	note := "auto-approved by the harness (measurement mode)"
+	if !ce.Exec {
+		a.store.Event(ctx, p.RunID, p.ScenarioID, "action", "ai-approved", map[string]any{"action_id": p.ActionID, "target": req.Target, "recorded_only": true, "note": note, "layer": ce.Layer})
+		mActs.WithLabelValues(p.ActionID, "ai-approved", "recorded").Inc()
+		_ = a.store.MarkProposalExecuted(ctx, token)
+		return
+	}
+	res, err := a.executeAction(ctx, p.ActionID, req.Target, "ai-approved", p.RunID, p.ScenarioID)
+	result := "ok"
+	if err != nil {
+		result = "error"
+	}
+	mActs.WithLabelValues(p.ActionID, "ai-approved", result).Inc()
+	a.store.Event(ctx, p.RunID, p.ScenarioID, "action", "ai-approved", map[string]any{"action_id": p.ActionID, "target": req.Target, "result": res, "error": errStr(err), "note": note})
+	_ = a.store.MarkProposalExecuted(ctx, token)
+}