feat: return bind credentials and reuse sync-bound agents on register (Z28)

Phone/bind-code/ticket responses include rotated client_secret; SelfRegister reuses active sync_bound hosts instead of spawning pending; archive coop opinion and keep a living summary.

Co-authored-by: Cursor <cursoragent@cursor.com>

platform/internal/logic/applogic/agents.go

@@ -146,7 +146,12 @@ func (l *AuthLogic) SelfRegisterAgent(req *types.AgentSelfRegisterReq) (*types.A
 	}
 	msg := "已登记为 pending，请管理员在控制台分配角色并启用后再换票"
 	if reused {
-		msg = "已存在 pending 登记，已轮换 client_secret；仍须管理员分配角色并启用"
+		syncBound := strings.TrimSpace(acc.ChannelID) != "" && strings.TrimSpace(acc.OnlineDBID) != ""
+		if acc.Status == agentstore.StatusActive && syncBound {
+			msg = "已存在同步绑定账号，已轮换 client_secret；请直接用返回的 client_id/client_secret 换票，勿再走绑定表单"
+		} else {
+			msg = "已存在 pending 登记，已轮换 client_secret；仍须管理员分配角色并启用"
+		}
 	}
 	return &types.AgentSelfRegisterResp{
 		Account:      *acc,