feat(deploy): 部署前校验 master.key 可解封配置 + llmsproxy -show-secrets

密钥校验（deploy.sh）
- 新增 verify_master_key，在 build/替换任何文件之前执行。失败则二进制与
  配置分毫未动、服务不受影响（已负向验证：缺钥匙、错钥匙两种情况都挡住）
- 走真实的 -show-secrets 解密路径，而不是只检查钥匙文件格式——格式合法
  但内容不匹配（重新生成、恢复了错的备份、换机器）同样会被拒
- 钥匙来源与 config 包一致：LLMS_PROXY_MASTER_KEY 优先，否则
  dirname(runtime_file)/master.key
- 配置里没有密文时跳过并提示（首次加密场景）

-show-secrets
- llmsproxy -show-secrets -config <path>：把凭据打到 stdout 后退出
- 不启动任何东西、不写任何文件（已验证 mtime 不变）
- 加密往返无损：封存前后输出逐字节一致

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/llmsproxy/main.go

@@ -24,6 +24,7 @@ import (
 func main() {
 	cfgPath := flag.String("config", "config.yaml", "path to gateway config file")
 	checkOnly := flag.Bool("check", false, "validate the config file and exit (0 = valid, 1 = invalid); nothing is started and no file is written")
+	showSecrets := flag.Bool("show-secrets", false, "print the config's credentials in the clear and exit; nothing is started and no file is written")
 	flag.Parse()
 
 	// -check is the deploy-time preflight: parse and validate the config
@@ -41,6 +42,20 @@ func main() {
 		return
 	}
 
+	// -show-secrets is the operator escape hatch for a config whose
+	// credentials are sealed at rest: it prints them to stdout and exits
+	// without starting anything or writing anything. Nothing else in the
+	// program prints a credential, and this path never logs to a file.
+	if *showSecrets {
+		if _, err := os.Stat(*cfgPath); err != nil {
+			log.Fatalf("[llmsproxy] show-secrets: %v", err)
+		}
+		if err := config.PrintSecrets(*cfgPath); err != nil {
+			log.Fatalf("[llmsproxy] show-secrets: %v", err)
+		}
+		return
+	}
+
 	created, err := config.EnsureDefault(*cfgPath)
 	if err != nil {
 		log.Fatalf("[llmsproxy] config: %v", err)