Log R4 reminder injection for observable evidence

Why: Reminders are attached to the request copy and never reach the
session journal, so a transcript grep proves nothing about whether the
R4 trigger fired - the first verification attempt was invalid evidence.
The dual-speed nudge already logs for exactly this reason.

Evidence: go test ./internal/runtime green; the follow-up live probe
must show the [runtime] r4 escalation reminder injected line in the
instance log.

Impact: Two log lines, nothing else.

Refs: Dev task feature/agent-e4-escalation-ledger (third fix-up).

internal/runtime/factcheck.go

@@ -3,6 +3,7 @@ package runtime
 import (
 	"encoding/json"
 	"fmt"
+	"log"
 	"regexp"
 	"strings"
 
@@ -343,9 +344,11 @@ func (st *directRun) escalationReminders() []string {
 	}
 	st.toolMu.Unlock()
 	if fails >= escalationFailureThreshold && !failNoted {
-		out = append(out, fmt.Sprintf("工具最近集中失败（近 %d 次调用中多次报错）：停止原样重试。逐条重读最近一次错误信息，改参数、换工具或换路径再试；确认无法绕过时，如实在答复里说明卡点与已尝试的手段，不要编造结果。", fails))
+		log.Printf("[runtime] r4 escalation reminder injected (tool failures %d in last %d outcomes)", fails, escalationWindow)
+		out = append(out, fmt.Sprintf("工具最近集中失败（近 %d 次调用中多次报错）：停止原样重试。逐条重读错误信息，改参数、换工具或换路径再试；确认无法绕过时，如实在答复里说明卡点与已尝试的手段，不要编造结果。", fails))
 	}
 	if zero >= escalationZeroHitThreshold && !zeroNoted {
+		log.Printf("[runtime] r4 escalation reminder injected (zero-hit searches %d)", zero)
 		out = append(out, "检索已连续多次无结果：换关键词/换检索维度再试一次；仍无结果就如实写「材料中无此信息」，不要凭印象补数字。")
 	}
 	return out