feat: agent 本机 SimAdmin 登录与会话维持；中继设备侧限流与 agent Retry-After

- runtime.NodeAuth：agent 自查 /api/auth/status，需要时自行 POST /api/auth/login 并持
  simadmin_session Cookie 转发；错误密码进入 60s 冷却（对齐控制台 ensureAuthenticated），
  节点回 401/403 即失效重探，SimAdmin 重启无需重启 agent
- 节点密码进 agent 配置（nodePassword），永不经中继、永不下发到手机；登录路径仍被策略
  挡在电话可达面之外（TestAuthPathsAreUnreachable）
- 新拒绝分类 node_auth_failed：与 upstream_error 区分，手机可提示'该节点需重新配置'
- 中继：/v1/request 每设备固定窗口限流（默认 30/分钟），未鉴权调用不消耗预算
- transport：解析 Retry-After 并入退避；main loop 对 429 直接遵从中继指示
- contracts/envelope.schema.json 补 InnerResponse.outcome 字段
- 设计文档与 README 同步：WSS 改为长轮询的实现修订、响应方向已临时×临时、P2/P3 状态

internal/transport/transport.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -236,6 +237,8 @@ func (c *Client) post(ctx context.Context, path string, body any, out any, sessi
 		// 204 and 408 are how the relay says "nothing yet", so the agent loops straight back.
 		return ErrNoWork
 	case http.StatusTooManyRequests:
+		retry := parseRetryAfter(response.Header.Get("Retry-After"))
+		c.retryAfter = retry
 		return fmt.Errorf("%w: retry after %s", ErrRateLimited, response.Header.Get("Retry-After"))
 	default:
 		return fmt.Errorf("transport: %s: relay returned %d", path, response.StatusCode)
@@ -263,6 +266,16 @@ func (c *Client) post(ctx context.Context, path string, body any, out any, sessi
 // ErrRateLimited is returned when the relay asks the agent to slow down.
 var ErrRateLimited = errors.New("transport: rate limited by the relay")
 
+// parseRetryAfter reads an RFC 9110 delay-seconds value. Dates are not supported and fall back to
+// the default: a clock-skewed node should not be waiting on an arithmetic date subtraction.
+func parseRetryAfter(value string) time.Duration {
+	seconds, err := strconv.Atoi(strings.TrimSpace(value))
+	if err != nil || seconds < 0 || seconds > 3600 {
+		return 2 * time.Second
+	}
+	return time.Duration(seconds) * time.Second
+}
+
 // RetryAfter reports how long the agent should wait before its next poll, honouring the relay's
 // Retry-After when it gave one.
 func (c *Client) RetryAfter() time.Duration {