feat(tools): add MessageTargetEnforced session guard helper

Classifies session kinds into FREE (cron, heartbeat, subagent, team) vs
ENFORCED (DM, group, ws, default, unknown, empty). FREE kinds compose
targets per job spec; everything else must match the ctx-bound origin.

postCrossTargetNotice posts the audit breadcrumb back to the origin
chat when an explicit forward succeeds: msgBus preferred, falls back to
direct sender, else slog.Warn so the skip is traceable.

internal/tools/message_guard.go

@@ -0,0 +1,73 @@
+package tools
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/nextlevelbuilder/goclaw/internal/bus"
+	"github.com/nextlevelbuilder/goclaw/internal/i18n"
+	"github.com/nextlevelbuilder/goclaw/internal/sessions"
+	"github.com/nextlevelbuilder/goclaw/internal/store"
+)
+
+// MessageTargetEnforced reports whether the message tool must enforce
+// (channel, target) == ctx (channel, target) for this session kind.
+//
+// FREE kinds (cron, heartbeat, subagent, team) compose targets per job spec
+// and are allowed to address arbitrary chats. Everything else (DM, group,
+// default, unknown, empty) enforces the bound origin — fail-closed.
+func MessageTargetEnforced(sessionKey string) bool {
+	if sessionKey == "" {
+		return true
+	}
+	if sessions.IsCronSession(sessionKey) {
+		return false
+	}
+	if sessions.IsHeartbeatSession(sessionKey) {
+		return false
+	}
+	if sessions.IsSubagentSession(sessionKey) {
+		return false
+	}
+	if sessions.IsTeamSession(sessionKey) {
+		return false
+	}
+	return true
+}
+
+// postCrossTargetNotice posts a short notice back to the origin chat after
+// an explicit, SUCCESSFUL cross-target forward. Call inline post-send — not
+// via defer — so that failed forwards don't announce fake deliveries.
+//
+// Prefers msgBus (async, group-aware). Falls back to t.sender so deployments
+// that wire only a direct sender still get the audit breadcrumb. Last resort:
+// slog.Warn so the skip is still traceable.
+func (t *MessageTool) postCrossTargetNotice(ctx context.Context, target, reason string) {
+	origCh := ToolChannelFromCtx(ctx)
+	origChat := ToolChatIDFromCtx(ctx)
+	if origCh == "" || origChat == "" {
+		return
+	}
+	locale := store.LocaleFromContext(ctx)
+	text := i18n.T(locale, i18n.MessageCrossTargetForwarded, target, reason)
+
+	if t.msgBus != nil {
+		msg := bus.OutboundMessage{Channel: origCh, ChatID: origChat, Content: text}
+		if isGroupContext(ctx) {
+			msg.Metadata = map[string]string{"group_id": origChat}
+		}
+		t.msgBus.PublishOutbound(msg)
+		return
+	}
+	if t.sender != nil {
+		if err := t.sender(ctx, origCh, origChat, text); err == nil {
+			return
+		} else {
+			slog.Warn("message.cross_target_notice_send_failed",
+				"channel", origCh, "chat", origChat, "err", err)
+			return
+		}
+	}
+	slog.Warn("message.cross_target_notice_skipped",
+		"reason", "no_bus_or_sender", "channel", origCh, "chat", origChat)
+}