fix(flow): 发布 backthink 会话的 graph_update——后台思考页链路导图不再是'等待事件'

FlowPublisher allow 谓词只允许 web/web:*，sess-backthink 被排除→backthink 的
输入→思考→输出图从不发布，后台思考页'查看链路导图'恒为'等待 graph_update 事件'。
- backthink.go: 导出 DefaultSessionID="sess-backthink"
- wire.go: 谓词追加 sessionID==backthink.DefaultSessionID（web + backthink 都发；sess-xxx 隔离仍过滤）
go build/vet/test 全绿; 已重建部署. 下次 backthink(5min心跳)运行时即发布该会话图.

cmd/cyrene/wire.go

@@ -323,7 +323,12 @@ func (a *app) wireRuntime() error {
 	flowPub := loop.NewFlowPublisher(
 		func(subject string, payload any) { _ = a.bus.Publish(context.Background(), subject, payload) },
 		func(sessionID string) bool {
-			return sessionID == history.MainSessionID || strings.HasPrefix(sessionID, history.MainSessionID+":")
+			// Publish the web/main flow graph (web chat + IM owner-private mirror) AND the
+			// autonomous-backthink session, so the backthink page's flow link graph renders
+			// too (design/20 §2). Other isolated channel sessions (sess-xxx) stay filtered.
+			return sessionID == history.MainSessionID ||
+				strings.HasPrefix(sessionID, history.MainSessionID+":") ||
+				sessionID == backthink.DefaultSessionID
 		},
 	)
 	checkpoint, perr := persist.New(context.Background(), a.db, a.log)