feat: 开源准备与 upgrade URL 构建时配置

- upgrade: SECRETS_UPGRADE_URL 改为构建时优先（option_env!），CI 自动注入
- upgrade: 支持运行时回退（.env/export），添加 dotenvy 加载 .env
- 泛化示例：IP/实例 ID/域名/密钥名改为示例值（10.0.0.1、example.com 等）
- tasks.json: 文件 secret 测试改用 test-fixtures/example-key.pem
- 文档更新：AGENTS.md、README.md

Made-with: Cursor

Cargo.lock

@@ -1836,7 +1836,7 @@ checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
 
 [[package]]
 name = "secrets"
-version = "0.9.1"
+version = "0.9.2"
 dependencies = [
  "aes-gcm",
  "anyhow",
@@ -1844,6 +1844,7 @@ dependencies = [
  "chrono",
  "clap",
  "dirs",
+ "dotenvy",
  "flate2",
  "keyring",
  "rand 0.10.0",